Color in Your Life

Citation preview

COLOR IN YOUR LIFE IRVING ADLER

Illustrated by RUTH ADLER

Media Center

N

Wfil. T. Rogers Middle School Mpr

?'*>

--

Basic structure and characteristics of color and light as well as more complex aspects, such as polarization, the Doppler effect, etc.

Wm. T. Rogers Middle School ied?.a Cep*

COLOR IN YOUR LIFE

Books by Irving Adler

COLOR IN YOUR LIFE

SEEING THE EARTH FROM SPACE

HOT AND COLD

WEATHER IN YOUR LIFE

DUST

THE TOOLS OF SCIENCE

THE SUN AND ITS FAMILY

MONKEY BUSINESS: HOAXES IN THE NAME OF SCIENCE

HOW LIFE BEGAN

MAGIC HOUSE OF NUMBERS

THE STARS:

STEPPINGSTONES INTO SPACE

TOOLS IN YOUR LIFE

FIRE IN YOUR LIFE

TIME IN YOUR LIFE

fp

COLOR IN YOUR LIFE BY

IRVING ADLER ILLUSTRATED BY

RUTH ADLER

fort S&iongM uviflciitdijr oo.iuui Liufary .. Kings Parki

THE JOHN DAY COMPANY NEW YORK

port. Salonga Elementary School Library Kings Park, N. Y*

/ o ' o C,

© 1962 by Irving and Ruth Adler All rights reserved. This book, or parts thereof, must not be reproduced in any form without permission. Published by The John Day Company, 62 West 45th Street, New York 36, N.Y., and on the same day in Canada by Longmans, Green & Company, Toronto.

Sixth Impression

Library of Congress Catalogue Card Number: 62-7784 Manufactured in the United States of America

CONTENTS

I

The World of Color

7

II

The Color of Light

13

III

The Color of Things

21

IV

Sifting out Colors

39

V

Broadcasting Color

57

VI

Nature’s Colors

67

VII

Improving on Nature

77

VIII

The Color We See

87

IX

Color in Printing, Television, and Photography

109

X

Interpreting Color

121

Index

126

I The World of Color The Color Around Us

\\J

e live

* *

in a beautiful world. A large part of

its beauty is made by color. Up above us,

we see white and gray clouds sailing across a blue sky. At sunset, the clouds sometimes become a fiery red, while the sky in the west becomes a deep purple. At night, if we look carefully at the twinkling stars, we can see that they, too, are colored. The star Capella is yellow, Arcturus is orange, and Antares is red. On the ground, we may see yellow daffodils, red roses, or blue cornflowers surrounded by green grass and shrubs. In the fall, before the leaves of trees fall off, they change their color from green to yellow and red and brown. At home, our walls are covered with colored paints or paper. Our furniture is covered with colored fab¬ rics. When we dress, we choose carefully the colors of our clothes. When we eat, we even choose the colors of our food, so that while our mouths enjoy the flavors, our eyes enjoy the colors.

7

Color is one of the languages of the artist. While the author talks to us with words and the musician talks to us with sounds, the painter talks to us with the colors of his palette. No Light, No Color Look at an outdoor scene that is rich in color and watch the colors as time passes. In the afternoon, when the scene is bathed in sunlight, the colors are bright and sharp.

At twilight, as the light grows dim, the

colors begin to fade. Then, at night, when the light of the sun is completely cut off by the ground, the colors vanish, and everything looks black or gray. This shows us that we cannot see color without the help of light. The Color of Things When we look at grass, the green color we see seems to be in the grass. When we look at an apple, the red color we see seems to be in the apple. When we look at the sky, the blue color we see seems to be in the sky. It looks as though everything has its own color, and the color is in the thing. that way at all.

But sometimes it doesn’t seem

If we look at a fabric in lamplight,

we may see one color. Then if we look at the same fabric outdoors, in daylight, we may see another color. This fact presents us with a puzzle.

Which color is

the real color of the fabric, the lamplight color or the

8

daylight color?

Or perhaps the color is not in the

fabric at all. Is the color in the fabric or in the light? No Eyes, No Color Look at a green lawn, and then close your eyes. As soon as your eyes are closed you stop seeing the lawn. And when you don’t see the lawn, you don’t see its green color. When you open your eyes, you see the lawn and the green color again. This shows that you cannot see color without the help of your eyes.

Light sourc Material medium

9

Color Without Things At night, when it is dark, close your eyes and press gently against one eyeball with your finger. You will see a circle of colored light in the eye that is being pressed. This color is not in a thing, because you are not looking at anything.

It is not in light, because

there is no light reaching your eye. The color seems to be in your eye. This fact gives us another puzzle to think about.

Perhaps all color is in the eye.

When

we look at green grass in daylight, is the green color in the grass, or in the light by which we see the grass, or in the eyes with which we look at the grass? Working Together Science has given us an answer to this question. The green color of grass is not in the grass alone, or in the light alone, or in the eyes alone.

It is produced by

all three: the grass, the light, and our eyes work to¬ gether to make the color we see.

In fact, there are

some other partners that share the work with them. To identify all the partners, let us trace the main happenings when we look at a colored object. They are shown in the diagram below.

First, light comes

from a light source. The light source may be a lamp or the sun, for example.

The light travels through

space to reach the object we are looking at.

On the

way to the object the light passes through whatever

10

material fills the space. It usually passes through air. If it is lamplight coming from a bulb or sunlight streaming into a window, it also passes through glass. After the light strikes the object, some of it travels from the object to our eyes. On the way to our eyes, it passes through a material medium again. Our eyes are connected to our brain. When light reaches the eyes, the eyes send messages to our brain.

So there

are at least six things that work together to produce the color we see. They are the light source, the light itself, the material medium through which the light travels, the object on which the light falls, the eye, and the brain. In the chapters that follow we shall examine what each of these things contributes to the work of producing the color we see. At the same time we shall learn many interesting facts about how colors are made and used.

11

N

II The Color of Light

Hidden Colors

r

I ^here are

colors hidden in ordinary sunlight.

We can bring them out of their hiding place with the help of a prism. If a narrow beam of sunlight is passed through a prism, the prism bends the beam and makes it fan out at the same time. The widened beam that comes out of the prism is made up of many narrow beams of light that are side by side. They are beams of colored light. The colors of these beams are violet, blue, blue-green, green, yellow, orange, and red. We see the colors when they are reflected to our eyes from a wall or screen. The beams of colored light are in the sunlight that enters the prism. We do not see the colors in ordinary sunlight that reaches our eyes because they are all mixed up in the sunlight. We do see them in the light that leaves the prism because the prism separates the colors.

It

separates

the

colors by bending

them

through different angles. The violet light is bent the

13

Red Orange Yellow Green Blue-green Blue Violet

most, and the red light is bent the least. The beam of separated colored lights, arranged in order from violet to red, is called the spectrum of the sunlight.

The

white light of the sun is a mixture of the colored lights of the spectrum. The Rainbow Nature sometimes separates the colors in sunlight all by itself. This happens at the end of a rain, when the sun is low in the sky. If you stand with your back to the sun, you may see a rainbow forming a big colored arch across the sky. The light of the rainbow is sun¬ light reflected to your eyes by small drops of water hanging in the air.

The drops of water behave like

prisms and separate the colors in the sunlight.

The

colors of the rainbow are the colors of the spectrum, arranged in the same order as they are in the spectrum. Sometimes we see rainbow colors indoors.

This

happens because there are things in our homes that act

14

like prisms, too. An empty milk bottle, a glass door¬ knob, the beveled edge of a mirror, or a glass bead on a chandelier can separate the colors in sunlight. Sun¬ light that passes through one of these things is often spread out into a spectrum on the floor or ceiling or a nearby wall. The Long and the Short of It Scientists have studied the colored light of the spec¬ trum for hundreds of years.

They have found that

each beam of light, no matter what color it makes us see, is made up of electrical and magnetic vibrations. They

call

these

vibrations

electromagnetic waves.

What makes the separate colored beams in the spec¬ trum different is the fact that they have different rates of vibration. The beams at the violet end of the spec-

Crest

wave length_^Crest

Trough

Trough

trum have faster vibrations than those at the red end. A beam of light of one color traveling through space behaves in many ways like a series of waves traveling over the surface of a lake. So we find it helpful to pic¬ ture the beam as a series of waves, as in the diagram above.

There are ups and downs in the electromag¬

netic vibrations just as there are in water waves. The

15

high points are called crests, and the low points are called troughs. The distance between two neighboring crests is called the wave length of the waves. Light waves of different colors have different wave lengths. In the spectrum the light waves are arranged according to the size of their wave lengths. The waves of red light are the longest, and the waves of violet light are the shortest. Light waves are very tiny.

In order to measure

them, we use a very small unit of measurement called the millimicron. A thousand million millimicrons is equal to one meter, or a little more than a yard. The symbol for a millimicron is m/a, the Latin letter “em” followed by the Greek letter “mu.” The shortest wave length of violet light is 390 m/a.

The longest wave

length of red light is 700 m/a. The wave lengths of the main colors of the spectrum are shown in this table: Color

Wave length

Violet

390 to 430 m/i.

Blue

430 to 460

Blue-Green

460 to 500 m/j-

Green

500 to 570

m/A

Yellow

570 to 590

m/A

Orange

590 to 610

m/A

Red

610 to 700

m/A

m/x

Frequency and Color All light waves travel through empty space at the

16

same speed.

This speed is about 186,000 miles per

second. So, when light of a single wave length passes by, the number of wave lengths that pass in a second is enought to stretch across a distance of 186,000 miles. This number is called the frequency of the wave. It takes more short waves than long waves to fill the same distance.

So the frequency of violet light is higher

than the frequency of red light.

The frequency of

violet light is about 750 million million waves per second. The frequency of red light is about 430 mil¬ lion million waves per second. Invisible Rays There are other rays in sunlight besides the colored light that we see in the spectrum. This is proved with the help of photographic film and a thermometer. If photographic film is held just outside a spectrum of sunlight at the violet end, it becomes exposed as if light were falling on it. This shows that there are in¬ visible electromagnetic rays there. We call them ultra¬ violet rays because they are beyond the violet end of the spectrum. If a thermometer is held on the other side of the spectrum, just past the red end, the mer¬ cury begins to rise. This shows that there are invisible rays there that are making the bulb of the thermom¬ eter warm. We call them infra-red rays because they are after the red end of the spectrum. Ultra-violet rays have a higher frequency and a shorter wave length than violet light. Infra-red rays have a lower frequency

17

and a longer wave length than red light. The ultra¬ violet rays in sunlight are the rays that cause sunburn. The infra-red rays in sunlight make much of the heat you feel when sunlight falls on your skin. One Big Family Ultra-violet rays, the colored light of the spectrum, and infra-red rays are all members of one large family, the family of electromagnetic rays.

There are other

members of this family, too. Among the rays that have a higher frequency and shorter wave length than ultra¬ violet rays are the X-rays a doctor uses to take pictures of the inside of your body. Among the rays that have a lower frequency and longer wave length than infra-

Electromagnetic radiation red rays are the radio waves that carry radio programs from broadcasting stations to your radio receiver. The many different electromagnetic rays are made in different ways. Just as there are special transmitters at a broadcasting station for making radio waves, there are special transmitters that make colored light. We shall find out in Chapter V what these transmitters for light are.

18

The many different electromagnetic rays are de¬ tected in different ways. Just as there are special radio receivers for detecting radio waves, there are special re¬ ceivers for detecting colored light. The eye is one of these receivers. Photographic film is another. The Spectrum of Lamplight •

The light that we use indoors at night is lamplight, usually made by a glowing tungsten wire. This light, like sunlight, is a mixture of colors.

A prism can

separate these colors to form a spectrum of the lamp¬ light. When we compare this spectrum with the spec¬ trum of sunlight, we find that they are not exactly the same. The chief difference is that lamplight has less blue and violet light in it than sunlight has. There are many different kinds of lamps in use today. Besides the tungsten lamp, which gives a pale yellowish light, there is the sodium-vapor lamp that gives a deep yellow light, the mercury-vapor lamp that gives a bluish light, and the neon lamp which gives a reddish light. The light from each of these lamps has its own spectrum, in which the colors are separated and lie side by side. The lights all look different be¬ cause they do not have the same amounts of the colors in their spectra.

19

Ill

The Color of Things Double Meaning

\\Te use a color name like “red” in two different " *

ways. Sometimes we use it to describe the

color of light in the long-wave end of the spectrum of sunlight. Sometimes we use it to describe the color of something we look at, like an apple or a rose. In this chapter we shall see how these two meanings of color names are connected.

A Three-Way Split To see the color of an object, we allow some sun¬ light to fall on the object. After the light falls on the object, some of it may travel on to reach our eyes. The color we see depends on what part of the light reaches our eyes. The sunlight is a mixture of the different kinds of light we find in its spectrum. Each of these kinds of light has its own color and wave length. Let us trace

21

what happens to one kind of light alone, with one color and one wave length. When light of one color and wave length falls on an object, the object divides the light into three parts. One part of the light is allowed to pass right through, as if the object were full of holes and the light went through the holes. We say that this part of the light has

been

transmitted.

Another

part of the light

bounces back from the object, the way a ball bounces back from a wall. We say that this part of the light has been reflected. A third part of the light is trapped in the object and is not allowed to escape. Usually the trapped light is turned into heat. We say that this part of the light has been absorbed. When light of one color falls on an object...

Different objects may divide the light in different ways. For example, one object may absorb all the red light that falls on it, and transmit and reflect none of it. Another object may transmit some and reflect some, but absorb none of it.

22

Each object has its own way of treating different colors or wave lengths.

Some objects are impartial:

what they do to one wave length they do to any other wave length. If they reflect one wave length, they re¬ flect all wave lengths. If they absorb one wave length, they absorb all wave lengths. Other objects pick and choose. They absorb some colors or wave lengths and reflect and transmit others. Passing Through Hold a glass of water before your eyes, and then hold things with different colors behind the glass. You will see each color through the glass of water as if the glass and the water were not there. The glass of water transmits nearly all of the light coming from each object. It transmits all of the colors equally well. Hold up a button behind the glass of water. Rays of light leave different parts of the button and move through the glass and water to your eyes. They leave in an orderly arrangement that matches the places they come from.

Light from the left side of the button

stays on the left. Light from the right side of the but¬ ton stays on the right. Light from the middle of the button stays in the middle.

When the light passes

through the water and glass, this arrangement is not disturbed. When the rays of light reach your eye, they make a picture of the button in your eye. This is why you can see the button clearly through the glass of water. Because you can see through it, we say that a

23

glass of water is transparent. Because all colors come through it equally well, we say that it is colorless. A thing is colorless and transparent if it transmits all

Rays of light leave the water in an orderly arrangement

Rays of light are scrambled by a glass of milk

colors impartially, and does not disturb the arrange¬ ment of rays that pass through side by side. Obstacle Course Hold a glass of milk before your eyes, and then hold up a button behind the glass. You will not be able to see the button through the glass. You may think that this means that light cannot pass through the milk.

24

To find out if it can or not, hold the glass of milk be¬ tween you and a lamp, and turn on the lamp. You will see the glow of the lamplight right through the milk. This shows that light does pass through the milk to reach your eye.

Then why can’t we see a button

through the milk? To answer this question, we must know what is in the milk. Milk is a mixture of many things. Most of the milk is water.

Some of it is butter fat, and some of it is

caseinogen, which is the stuff that cheese is made of. Tiny drops of butter fat and caseinogen are scattered throughout the water of the milk. Each drop is a little ball, so a glass of milk is a collection of millions of little balls floating in water. The balls are obstacles in the path of any light that is passing through. The light bounces from ball to ball, zigzagging between them as it moves along. The ball tosses the light around like boys tossing around some luckless fellow’s hat. Finally, when the light does escape from the milk, it may no longer be moving in the direction that it followed when it entered the milk. Rays of light that enter the milk in an orderly arrangement, side by side, are thor¬ oughly scrambled before they escape. When rays of light come from a button, they are arranged like the parts of the button. This is why they can make a pic¬ ture of the button in your eye. But when the rays are scrambled, the picture is destroyed. That is why you cannot see the button through a glass of milk. Since you cannot see through milk, milk is not transparent.

But light can pass through milk, so we

say that the milk is translucent.

25

The Color of All Colors Water is colorless and milk is white. This gives us a chance to understand the meaning of white.

Both

water and milk allow light to pass through them. Water allows light to pass straight through without disturbing the order of rays moving side by side. It transmits all colors equally.

Milk scatters the rays and scrambles

them. It scatters all colors equally. The scattering is responsible for making the milk look white.

The

paper on which this book is printed also looks white. You see the paper by light that is reflected from the paper. The paper reflects all the light that falls on it. It reflects all colors equally well, so they come to your eye from the paper mixed together, just as they are in sunlight. The paper is made of tiny threads whose faces are turned in all directions. The threads scatter the light the way the drops floating in milk do. The scattering is responsible for making the paper look white.

We say something is white if it does three

things: First, it transmits or reflects all the light that shines on it. Secondly, it treats all colors impartially. Third, it scatters the light at the same time.

(An ex¬

ception to this rule is discussed in Chapter VIII.) The Color of No Colors There are some things that absorb all of the light that falls on them, and absorb all colors equally well. Because they absorb all of the light, none of the light

26

shining on them can reach our eyes. Things like this look black. We say something is black if it does not reflect any light.

While a thing that is white sends

us all colors, mixed and scattered, a thing that is black sends us no colors at all. (An exception to this rule is discussed in Chapter VIII.) Between Black and White There are some things that reflect only part of the light that shines on them, but they treat all colors im¬ partially. These are the things that look gray. We say something is gray if it is an impartial reflector that is not as good as a white body, or as bad as a black body in reflecting the light that strikes it.

(An exception to

this rule is discussed in Chapter VIII. There we shall see that a gray body sometimes looks black or white.) Things That Pick and Choose Many churches have stained-glass windows. In these windows there are pictures made out of small pieces of colored glass arranged side by side. People who are inside the church see the pictures with the help of sunlight that falls on the window from the outside. A small part of the light is reflected by the window, so it never gets into the church.

The rest is either ab¬

sorbed or transmitted. Each piece of glass absorbs some colors, and allows the rest to go through.

Only the

colors that pass through the glass can reach the eyes of

27

N

the people in the church. The color they see in each piece of glass is the color of the light mixture that comes from that piece. The diagram below shows how different kinds of glass get their color by choosing the light that they The light strikes each glass from the left-hand side

Violet 1 Blue Green l Yellow C Red Green glass

Violet l.i.

Blue

JL T

mmmm Colored glasses act as filters

Green 1 Yellow C

1

Red Purple glass

28

allow to pass through. Ordinary sunlight is falling on each piece of glass from the left-hand side. Since sun¬ light is a mixture of many colors, the diagram shows these colors as separate bars. Five bars are shown for the main colors, violet, blue, green, yellow, and red. The amount of each light that passes through is shown by the length of the bar on the right-hand side of the glass.

The first glass absorbs all of the violet, blue,

green and yellow light, and allows only the red light to pass through.

So this glass looks red.

The second

glass absorbs all of the yellow and red light. It absorbs nearly all of the green light, and allows most of the violet and blue light to go through. This glass looks blue. The third glass absorbs all of the violet and blue light, and allows all of the red and yellow light and some of the green light to pass through.

This glass

looks yellow. The fourth glass absorbs all of the red and violet light. It allows most of the green light and a small part of the blue and yellow light to pass through.

This glass looks green.

The fifth glass ab¬

sorbs all of the green and nearly all of the blue light. It allows some violet light, most of the yellow light, and all of the red light to pass through.

This glass

looks purple. Things like colored glass that transmit light but pick and choose the colors that they allow to pass are called filters.

Most filters are made of glass, gelatin, or

plastics. Things That Block Light Most of the things we see around us do not allow

29

light to pass through them at all. Such things are said to be opaque. All the light that falls on an opaque object is either absorbed or reflected.

If the object

absorbs some colors and reflects others, then it looks colored. The color of an opaque object is the color of the light it reflects, just as the color of a filter is the color of the light it transmits.

A thing that reflects

mostly red light has a red color. A thing that reflects mostly blue and violet light has a blue color, and so on. Filtering More Than Once Suppose we make a filter out of the material of the purple filter shown in the diagram on page 28, but we make it twice as thick. Then let light pass through the double thickness.

Passing through the double thick¬

ness is like passing through two single-thickness filters, one after the other. The first filter is the first half of the double thickness. The second filter is the second half. The first filter removes all of the green light, so

Five thicknesses

Double thickness of purple filter .

Red filter

Orange filter

30

no green light comes through the double thickness. The first filter also removes most of the violet and blue light, and the second filter removes most of what is left. So almost no violet or blue light comes through the double thickness. The first filter removes a small part of the yellow light, and the second filter removes a small part of what is left.

So yellow light comes

through the double thickness, but in a smaller amount. Both filters allow all of the red light to come through. As a result, the light that comes through the double thickness is mostly red, mixed with some yellow. Such a mixture looks orange. So a double thickness of the purple filter is an orange filter.

If we use five thick¬

nesses, so much of the yellow light is removed that the light that comes through is almost pure red.

So a

fivefold thickness of the purple filter is a red filter. When we change the thickness of a filter we change its color. Reflecting More Than Once A similar change in color is caused by reflecting light from a colored surface more than once.

If the

surface absorbs some of the light of a certain color at each reflection, the more times the light is reflected, the more of that color is removed. After many reflec¬ tions, that color will be almost completely removed from the light.

For example, the color of gold is

usually orange, because when gold reflects sunlight, it removes all the blue and green light and some of the yellow light. But a golden goblet looks blood red in-

31

Vio Bli

'

Gr< Yellow Red

A gold coin reflects red and some yellow. It ab¬ sorbs all other colors.

side. This is because light coming from the inside of

the goblet is reflected back and forth inside the goblet before it reaches our eyes. some of the yellow light.

Each reflection removes

After many reflections, sc

much of the yellow light has been removed that what is left is almost pure red. Inside a Paint

A paint is made by mixing a fine powder with < transparent, usually colorless, liquid.

The way ir

which a paint colors a wall that it is painted on depend; on the kind of mixture it is.

In some paints, the particles of powder are colorec

and transparent. The powder is called a pigment. Ir these paints, each particle of pigment acts like a smal filter. Light shining on the painted surface first passe

through the particles. Then it strikes the wall behint

the paint. Then, if some light is reflected from th< 32

wall, it passes through the particles a second time on the way out of the paint. The light that finally reaches your eyes from the wall may have colors removed from it three times.

Colors are removed by the filtering

action of the particles before the light reaches the wall. Colors are removed when the light strikes the wall and is reflected.

Then colors are removed again by the

filtering action of the particles after the light is re¬ flected.

The color you see on the wall depends on

what colors are left in the light that reaches your eyes. In this case the wall as well as the paint has a share in making the color that you see. In some paints, the particles of powder are colored and opaque. This type of powder is also called a pig¬ ment.

In these paints, each particle acts like a small

reflector. Light striking the painted surface is reflected by the separate particles. It may be reflected from one particle to another many times before it leaves the paint and comes to your eyes.

If there are enough

particles in the paint, and the paint is thick enough, the light will never get through to the wall behind the paint. So the wall has no share in making the color you see. In a third type of paint, the particles of powder are white and opaque, but each particle is surrounded by a layer of transparent dye. A powder like this is called a lake. When light enters this kind of paint and strikes a particle, it passes through the dye that is on the outside of the particle, and is reflected by the white material that is on the inside. 33

In all three kinds of paint, the light is reflected or transmitted by the small particle many times before it reaches your eyes.

Changing the amount of powder

in the paint or changing the thickness of a layer of paint changes the number of times the light is reflected or transmitted.

As we have seen, this may have the

effect of changing the color of the light that finally comes out of the paint. So one single paint may give you many different colors, depending on how the paint is mixed and applied. A fourth type of paint is made by mixing a white, opaque powder with a liquid that is transparent and colored.

The color of the liquid comes from a dye

that is dissolved in it. The solution may be dilute (a small amount of dye in a large amount of liquid), or it may be concentrated (a large amount of dye in a small amount of liquid).

In this type of paint, the

liquid, instead of the powder, acts as a filter. A con¬ centrated solution acts like many filters that are one behind the other.

So changing the concentration of

the dye can change the color of the paint. For example, there is a dye that is green in a dilute solution, but a concentrated solution of the same dye is red.

A Red Ink That Prints Blue If a pigment absorbs a color in large amounts, it also reflects some of that color. As a result, the light the pigment reflects contains the very colors that are missing in the light it allows to pass through. Because 34

of this fact, a pigment like this has two different colors. One is the color you see if you receive the light it re¬ flects.

The other is the color you see if you receive

the light it transmits. An interesting example of this is a red printers’ ink that prints blue. When you look at a big lump of the ink, it looks red because you see it by reflected light.

When the ink is printed on a white

page, only a thin layer of the ink is spread out over the page. The light you see then is light transmitted through the ink and reflected back by the page. This transmitted light has the red light missing, and looks blue. Wet Sidewalks You have probably noticed that a sidewalk looks darker when it is wet than when it is dry. What you have learned about absorption and reflection can help you understand why. When you look at a sidewalk, you see it by the light that it reflects to your eyes. This light is weaker than the light that falls on the sidewalk, because the sidewalk absorbs some of the light. That is why the sidewalk looks gray. When the sidewalk is wet, there is a layer of water on it. The light reflected by the sidewalk passes through the water first on the way to your eyes. But the surface of the water acts like a mirror. It reflects some of this light back into the sidewalk, giving the pavement a chance to absorb some more of the light. The light that finally comes out has been weakened more than once, so the side¬ walk looks a darker shade of gray. 35

When Blue Looks Black When we say that a pigment is blue, it does not mean that it always looks blue. It means that the pig¬ ment looks blue when it is seen by ordinary sunlight. If it is looked at in some other light, it may be a differ¬ ent color altogether. For example, there are pigments that look blue in sunlight because they absorb the yellow and red light in the sunlight and reflect only the violet, blue and green light.

If we shield such a

pigment from the sunlight and shine a pure yellow light on it, the pigment absorbs all of the yellow light and reflects none of it.

It cannot reflect blue light

then, because it does not receive any blue light in the first place.

Since our eyes receive no light from the

pigment at all, the pigment looks black. The table below shows how some pigments “change color” when we change the light by which we see them. The first column shows the color of the pigment in white light, or ordinary daylight. We usually speak of this color as if it belongs to the pigment. But it really belongs more to the light that shines on the pigment than to the pigment itself. Strictly speaking, the pig¬ ment doesn’t have any color of its own. It only has the ability to pick out colors from the light that falls on it, and send them to our eyes. The important thing to keep in mind is that it cannot pick out a color that isn’t there. 36

Colors of Some Pigments When Viewed in Different Kinds of Light

When seen by When seen by When seen by When seen by blue-green light mauve light white light yellow light Blue

Black

Blue

Purple

Yellow

White

Green-yellow

Orange

Green

Green

Yellow-green

Black

Red

Red

Black

Scarlet

Magenta

Red

Blue

Pale Magenta

The light of a tungsten lamp has less blue light in it than daylight does.

For this reason, pigments and

dyes do not look the same by lamplight as they do by daylight. This is a good thing to keep in mind when you buy your clothing.

The color of the cloth that

you see in the store may not be the same as the color of the cloth in daylight. Examine the cloth in daylight to be sure that the color is the one you really want. Making a Scene Disappear You can change the color of an object by changing the light that shines on it. This makes it possible for show people to make one costume do the work of many. 37

An actress appears on the stage in one costume, and a colored spotlight is shone on her. When the color of the spotlight is changed, the color of the costume changes.

By using four different spotlights, she can

be made to wear four different costumes without even changing her clothes once. Colored spotlights make it possible to change the scene on a stage without shifting the scenery at all. The chart on page 37 shows us how.

For example,

two scenes might be painted side by side on a black background. One of them is painted red and the other one is painted blue. When a yellow spotlight is shone on the stage, the red scene looks red and the blue scene looks black.

But black cannot be seen against

a black background.

So the blue scene disappears.

When a blue-green light is shone on the stage, the blue scene looks blue and the red scene looks black. So then the red scene disappears.

38

IV Sifting Out Colors

Sieves for Colors

O oil

is a mixture of particles of many sizes. It

^ includes small particles, like powdery clay, and larger particles, like pebbles.

It is possible to

separate the small particles from the large particles by passing the soil through a sieve. If the right sieve is used, the large particles will be too large to go through the holes of the sieve, while the small particles will pass right through. Sunlight is a mixture of light of many wave lengths. There are long wave lengths (the red light), and short wave lengths

(the violet and blue light).

Just as a

sieve can separate soil particles by size, there are special “light sieves’’ that can separate the colors in sunlight by wave length. We saw in Chapter II that a prism is like a light sieve, because it separates the colored rays in the sunlight by bending them. In this chapter we shall look into the reason why it does so. We shall also learn how colors can be separated by such different 39

\

things as smoke rising from a chimney, soap bubbles, and the scratches on an LP phonograph record. The color filters described in Chapter III are also examples of “light sieves” because they absorb some wave lengths, while they transmit others. Opaque ob¬ jects are light sieves when they absorb some colors and reflect others. In Chapter V we shall find out why they act the way they do.

Running a Race The different colors in sunlight are racing against each other. As long as they move through a vacuum, or empty space, the outcome of the race is a tie, because all the colors move through empty space at the same speed.

But as soon as they move into a material

medium like air or water or glass, some colors get ahead of the others. This happens because a material medium slows the colors down and slows some colors down more than others. A prism takes advantage of this fact in order to separate the colors from each other. Some materials slow light down more than others do.

For example, light travels more slowly through

glass than it does through water, and it travels more slowly through water than it does through air.

To

show the slowing effect that a medium has on light, physicists use a number called the refractive index. They calculate it by dividing the speed of the light in a vacuum by the speed of the light in the medium. The average refractive index of some media for the 40

mixture of colors in sunlight is shown in this table: Refractive Index

Medium

Vacuum

1.000

Air

1.003

Water

1.333

Alcohol

1.361

Linseed Oil

1.49

Glass

1.50 to 1.96

Diamond

2.47

According to this table, sunlight travels 1.333 times as fast through a vacuum as it does through water.

It

travels 1.49 times as fast through a vacuum as it does through linseed oil, and so on. Many different indices are listed for glass, because there are many different kinds of glass. Each kind of glass has its own refractive index for sunlight. The figures shown in the table are averages for all the different colors. If the index is calculated for each color separately, we get a different number for each wave length.

In most materials, the shorter wave

lengths are slowed down more than the longer ones. So the refractive index for violet light

(short wave

length) is usually higher than the refractive index for red light (long wave length). Making Light Turn Suppose that a beam of light of a single wave length moves from air into glass, approaching the surface of

41

the glass at a slant. In the drawing below, the dotted lines represent the crests of the waves in the beam. The arrow shows the direction in which the waves are moving. Because a wave approaches the surface of the glass at a slant, it does not enter the glass all at once. In this case the lower part of the wave enters first. Since light travels more slowly in glass than in air, the lower part of the wave begins to fall behind the upper part of the wave. This makes the wave swing around as it enters the glass. So, after the whole wave is in the glass, it moves off in a new direction. The change in direction of the beam is called refraction.

Refraction of a beam of light

42

When a beam of sunlight passes from air into glass, each separate color in the sunlight is turned in this way. The short-wave light is slowed down more than the long-wave light. As a result the short-wave light is turned more than the long-wave light.

So, although

the different colors in the sunlight were traveling in the same direction while they were in the air, they travel in different directions after they enter the glass. In other words, the passage from air into glass separates the colors by wave length. A similar separation takes place any time sunlight crosses at a slant the boundary between two materials that have different indices of refraction. When sunlight passes through a prism, the bending of the light and the separation of colors takes place twice. It happens first when the light enters the glass from the air. It happens again when the light leaves the glass on the other side of the prism to return to the air. Making Light Bounce When a beam of light strikes the boundary between two materials that have different indices of refraction, only part of the light passes through and is refracted. Another part of the light bounces back or is reflected from the boundary. If the boundary is flat, the light bounces back in a single direction. What this direc¬ tion is depends on the angle at which the light ap¬ proaches the boundary.

43

If two materials that lie side by side have the same index of refraction, and light passes from one material into the other, then the boundary between them has no effect on the light at all. It doesn’t bend the light, and it doesn’t reflect the light.

Reflection from a flat surface

The light continues

curved surface

moving along in a straight line as though the boundary were not there. Scattering Light Sometimes the boundary between two materials that have different indices of refraction is curved. A curved surface may be thought of as being made up of many small flat pieces, each facing in a different direction. When a beam of light approaches a curved boundary, it strikes each little flat piece at a different angle. So each piece reflects the light in a different direction. As a result, a curved boundary scatters the light, or reflects it in many directions. A cloud floating in the air is made up of many small drops of water. Each drop has a curved surface, so it scatters the sunlight that strikes it. It scatters all colors in the sunlight equally.

When our eyes receive the

scattered light with all colors mixed in it as they are

44

Sunlight

Bec*

„

Orange ,

m

«

-"~~ '

^'fk-^BUtK* Blue

;*- T £? -k

For someone here, most of the blue light has already been scattered. But the red light comes through.

Green Blue-green

*1

Violet

For someone here, scattered blue and violet light reach the ground. So the sky looks blue.

in sunlight, we see the cloud as something white. We have already seen that a glass of milk looks white for the same reason. Particles or drops that float in the air scatter all colors equally only if they are much larger than the wave length of the light that strikes them. If they are about the same size as the light waves, then the scat¬ tering is unequal. Then they scatter the short-wave or violet and blue light more than they scatter the long-wave or red light. Many dust particles in the air, and the molecules of air themselves, are just the right size to do this unequal scattering.

So the air above us, when it is struck by

sunlight, scatters light that is mostly violet and blue in all directions. That is why the sky looks blue. Some of this scattered light reaches us on the ground. It is

45

the daylight that reaches into places that are in the shade.

Daylight that conies from the sky has more

violet and blue light in it than direct sunlight has. When the sun is close to the horizon, sunlight has to travel through more air than usual in order to reach us on the ground. As more and more blue light is scat¬ tered, there is less and less of the blue light left in the direct rays of the sun. By the time the rays reach the ground, so much blue light has been removed that what is left looks orange or red. That is why clouds in the western part of the sky turn orange or red at sunset. The Color of Smoke Smoke rising from a chimney is made up of small particles that scatter light the way the molecules of air do.

They, too, scatter blue light more than they

scatter red light. As a result, the smoke looks colored. But the color of the smoke depends on where you stand.

If you see the smoke by reflected light, the

smoke has a bluish color. But if the smoke is between you and the sun, you see the smoke by the sunlight that passes through the smoke.

This light, like sun¬

light at sunset, has a reddish color. If some milk is mixed with water, the little drops in the milk are broken up into smaller drops.

The

smaller drops then behave like molecules of air or particles of smoke. They scatter blue light more than yellow or red light.

That is why milk mixed with

water has a bluish color.

46

This person sees the red light that passes through the smoke.

This person sees the reflected blue light. So the smoke looks blue.

In a fog, there are many small drops of water floating in the air, as in a cloud. When an automobile head¬ light shines through a fog, so much of the beam is scattered by the water drops that it cannot reach very far. That is why you cannot see far ahead when you drive through a fog.

The fog doesn’t scatter yellow

light as much as it scatters blue light, so a beam of yellow light can reach farther in a fog than an ordinary headlight beam does.

That is why yellow lights are

often used as fog lights on a car. A Filter Made of Beads An interesting filter, known as a Christensen filter, is made by mixing small colorless beads with a color-

47

less liquid. The material of the beads and the liquid are chosen so that they have the same index of refrac¬ tion for light of one special wave length, but have dif¬ ferent indices of refraction for other wave lengths. If a beam of sunlight enters the mixture, the surfaces of the beads scatter the light of all wave lengths except the special one for which the beads and the liquid have the same index of refraction.

For this special

color, the surfaces behave as if they were not there, and the light passes right through the mixture. In this way, the special color is separated from all the rest. Some Christensen filters are made by putting glass beads into a mixture of benzene and carbon disulfide. The color that the filter allows to pass through can be changed by using more benzene and less carbon di¬ sulfide, or vice versa. When Light Destroys Light When you add two beams of light of the same color, sometimes they combine to form a stronger beam. But sometimes they do the very opposite. The beams de¬ stroy each other and there is no light at all.

This

strange result is made possible by the fact that each beam is a series of waves. A crest of a wave is a place where the electrical force that the wave carries pushes in one direction. A trough is a place where the elec¬ trical force pushes in the opposite direction.

If the

two beams that are added are in step with each other, each crest of one is combined with a crest of the other. The forces at the two crests push in the same direction,

48

Two beams combine to form a stronger beam

(No wave at all)

Two beams combine to give no light at all

so they strengthen each other. In this case, the beams combine to form a stronger beam.

But suppose the

two beams are out of step, with one beam half a wave length behind the other. Then each crest of one beam is combined with a trough of the other. The forces at a crest and a trough push in opposite directions, so they interfere with each other.

In fact, if the pushes are

equal, they wipe each other out. In this case the two beams combine to give no light at all. Splitting a Beam The fact that light waves can interfere with each other gives us a way of taking a single light beam that is moving in one direction, and splitting it into several beams moving in different directions. The device wth which we do the splitting is called a diffraction grating.

49

The grating is a flat glass plate with many thin straight parallel lines scratched on it. The scratches are made so close together that there are thousands of them to an inch.

The diagram below shows what happens

when light of a single wave length passes through the grating.

The parallel lines to the left of the grating

are the crests of waves approaching the grating.

As

we know, the distance from one crest to the next is the wave length of the light.

Halfway between any two

neighboring crests is a trough. After the light strikes a scratch on the grating, the scratch scatters the light. The light that leaves the scratch travels in all direc¬ tions from the scratch as if the scratch were a glowing

How a diffraction grating works

50

wire. So the crests that leave the scratch have the form of circles that grow wider as they move away from the scratch. As a result, crests and troughs coming from different scratches cross each other on the way. Where a crest crosses a crest, the light is strengthened. Where a crest crosses a trough, the waves interfere with each other, and the light is destroyed.

There are several

beams of strengthened light, shown by the arrows pointing away from the grating. Some diffraction grat¬ ings are made by scratching parallel lines on a mirror. This type of grating splits the light it reflects into several beams.* Using Scratches as a Sieve If the light that enters a grating is a mixture of many colors, each color is split into several beams going off at different angles. The angles at which the split beams leave are not the same for all colors. So the different colors, when they leave the grating at an angle, do not leave together. They come out side by side, arranged as in a rainbow, or in the spectrum formed by a prism. Because each color is split into several beams, there are several spectra formed in this way. There are many common objects that behave like diffraction gratings and separate sunlight into the rain¬ bow colors. A long-playing phonograph record is one of them. It has many fine grooves on it which carry the sound track.

If you hold a record so that it reflects

* For more details about diffraction gratings, see The Tools of Science, by Irving Adler, The John Day Company, 1958.

51

sunlight or lamplight to your eyes, the grooves act like a grating and separate the colors in the light. As you tilt the record back and forth, rainbow colors glide along the grooves. If you close your eyes almost all the way and look at the light through your eyelashes, you see rainbow colors too. This is because the narrow spaces between your lashes behave like the scratches on a grating. An¬ other common “grating” that separates colors in the same way is a thin woven cloth, like muslin, for ex¬ ample. If you look at the sun through the cloth, you see a patchwork of colors. Colored Soap Bubbles When you blow soap bubbles, sometimes the bub¬ bles are brightly colored. The color of the bubble is the result of some reflected rays of light interfering with each other. The soap bubble has two surfaces, one on the outside and one on the inside. When light of a single wave length approaches the bubble, some of it is reflected back from the outside surface.

The

rest passes through the skin of the bubble. Then some more of the light is reflected back from the inside sur¬ face. The light reflected from the inside surface trails behind the light reflected from the outside surface for two reasons. First, it had to travel some extra distance through the skin of the soap bubble and back.

Sec¬

ondly, when it is reflected at the inner surface, it is held back half a wave length. If the total distance it falls behind is 11/2, or 21/2, or 31/2 wave lengths, and so on, then the two reflected beams are out of step. Crests

52

If the thickness of the soap film is one-half wave length, the crests meet troughs and can¬ cel each other

Why soap bubbles are colored meet troughs and cancel each other. When daylight, which is a mixture of many colors, is reflected from a bubble, some of the colors in the reflected light are canceled in this way, while others are not. The color of the bubble is the color of the light that is not can¬ celed. A Spigot for Light To help us understand the way light waves behave, we have pictured the wave as a wavy line drawn on a plane or flat surface. Actually, a picture like this rep¬ resents only a special kind of light, in which the elec-

53

trical vibrations are back and forth in one plane. Light like this is known as polarized light.

Ordinary sun¬

light is a mixture of many rays of polarized light, vi¬ brating in many different planes. We can get polarized light from sunlight by passing it through some special materials such as a Nicol prism. The prism behaves as though it had a slit in it which permits only vibrations in the plane of the slit to pass through while the other vibrations are blocked.

Two Nicol prisms together

can serve as a spigot for light. After light has passed through the first prism, it is polarized. If the second prism is in the right position, so that the plane of its imaginary “slit” matches the plane in which the polar¬ ized light vibrates, the light also passes through the second prism. If the second prism is rotated gradually, less and less of the polarized light passes through, until finally after the prism has been turned through ninety degrees, the light is cut off altogether. So turning the second prism is like turning a spigot for controlling the flow of light. Giving Light a Twist If polarized light is passed through a thin sheet of quartz, it gets through to the other side, but it is changed on the way through.

The quartz gives the

light a twist, so that the plane of its vibrations is turned. The amount of the turning depends on the thickness of the sheet.

It also depends on the wave length or

color of the light.

For example, if the sheet is one

thousandth of a meter thick, it turns the plane of vi¬ bration of the light through these angles: red light, 18

54

degrees; orange light, 22 degrees; yellow light, 24 de¬ grees; green light, 30 degrees; blue light, 32 degrees; violet light, 42 degrees. If this sheet of quartz is placed between two Nicol prisms, we have a spigot from which we can pour colors, and choose the color we get.

If

sunlight is passed through the combination, the first prism polarizes all the colors in the sunlight.

The

quartz sheet separates the colors by rotating them through different angles.

Then the second prism al¬

lows only one color through at a time. If we turn the second prism 18 degrees from its normal position, it lets red light through.

At 22 degrees, it lets orange

light through, and so on. There are many minerals that can twist polarized light the way quartz does. A pair of Nicol prisms can be used to measure the amount of twisting that takes place. This information is a clue that helps to identify the minerals. Changing the Color of Light Sometimes, when you are out riding in a car, you pass another car that is honking its horn in the oppos¬ ing lane. You may have noticed that the sound of the horn seems to change as you pass the car. While you are moving toward the car, the sound has a high pitch. After you have passed the car and you are moving away from it, the pitch suddenly drops. This change in pitch is known as the Doppler effect.

There is a

Doppler effect on light, too. What is a change in pitch for sound, becomes a change in color for light.

If a

light source sends you light waves of a certain fre-

55

quency and you receive them while standing still, the number of waves that reach you in a second is the frequency of the light.

If you rush forward to meet

the light, you receive more than this number of waves in a second. The result is that the light you see has a higher frequency, and is closer to the violet end of the spectrum of sunlight. If you run away from the source of light, you receive fewer than this number of waves in a second. The result is that the light you see has a lower frequency, and is closer to the red end of the spectrum. Astronomers have found that the light of distant galaxies shows this kind of shift toward the red. They interpret this to mean that the galaxies are moving away from us at high speed. The Doppler effect is the subject of a joke that physi¬ cists tell.

It seems that a man once got a ticket for

passing a red light.

When he appeared before the

judge, he tried to talk his way out of his predicament. He told the judge all about the Doppler effect and said that because of the Doppler effect, the red light looked green as he approached.

Since the light he

saw was green, he wasn’t guilty of passing a red light, he argued. The judge did some writing on a piece of paper and then said,

“I

accept your explanation.

Twenty-five dollars fine.” The man demanded to know why he was fined, if the judge accepted his explanation. Then the judge explained, ‘‘If you are telling the truth, you are not guilty of passing a red light.

But

then you are guilty of speeding. I just figured out that in order to make a red light look green you would have to travel at one third the speed of light, or about 62,000 miles a second.”

56

V Broadcasting Color

Transmitters for Light

r I ’’he waves that carry the radio programs that we listen to are electromagnetic waves. They are sent out by special radio transmitters at the broad¬ casting stations. Light waves are also electromagnetic waves. They, too, are sent out by special transmitters. The transmitters which broadcast light are the tiny atoms and molecules of which all things are made. Every common object, whether it is a gas, a liquid, or a solid, is made up of molecules. There are hun¬ dreds of thousands of different kinds of molecules. Each molecule is made up of atoms. There are about one hundred different kinds of atoms. Each atom, in turn, is made up of smaller particles called protons, neutrons, and electrons.

The protons and neutrons

are packed together in a nucleus, and the nucleus is surrounded by the electrons.

Atoms differ in the

number of protons that are in the nucleus. If an atom is complete,

the number of electrons around the

57

N

nucleus is equal to the number of protons inside the nucleus. Chemists use a special symbol to stand for each kind of atom.

The letter H stands for a hydrogen atom.

The symbols C, O, S, N, Cr, and Fe stand for atoms of carbon, oxygen, sulphur, nitrogen, chromium, and iron, in that order.

To represent a molecule, they

write the symbol for each kind of atom in the mole¬ cule, and attach numbers to show how many atoms of each kind are in the molecule. For example, HaO stands for a molecule of water, because a molecule of water contains two hydrogen atoms joined to an oxygen atom. There are some groups of atoms that do not make a complete molecule but are often found joined to¬ gether within a molecule, and usually act together as a unit.

These groups are called radicals.

Some ex¬

amples that we shall meet later are the hydroxyl radical (OH), which contains one oxygen atom and one hydro¬ gen atom, and the chromate radical (Cr04), which con¬ tains one atom of chromium and four atoms of oxygen.

Shells and Steps The electrons that surround the nucleus of an atom are arranged around it in separate layers or shells. The first, or innermost shell, has room for two elec¬ trons.

The second and third shells have room for

eight electrons each. A hydrogen atom has only one electron, and it is in the first shell. A carbon atom has

58

six electrons. Two of them are in the first shell, and the other four are in the second shell. When an atom sends out light, it is the electrons in the outermost shell of the atom that do the broadcasting. Each electron is held in place near the nucleus by an electrical pull of the nucleus, in the same way that things on the earth are held in place near the earth by gravity, or the pull of the earth. Things at rest near the earth may be at different levels above the ground. In the same way, an electron may be at different levels above the nucleus. There are only certain fixed levels at which an electron may stay.

It is convenient to

picture these levels as a flight of steps.

An electron

may move up or down from the top of one step to another, but cannot stay between steps. When an Electron Falls To raise an electron from a lower step to a higher step, it is necessary to oppose the pull of the nucleus. Energy must be supplied to raise an electron to a higher step. This energy is stored in the electron. The

Hydrogen atom

59

higher the step it is on, the more energy there is stored in the electron. When one of the electrons of an atom is raised from its normal position to a higher step, we say that the atom is in an excited state. After an elec¬ tron has been raised to a higher step, it promptly falls back to a lower step. When it falls, it releases some of the energy that was stored in it. The amount of energy released is the difference between the amount stored at the upper level and the amount stored at the lower level. The energy that is released goes off in the form of a small bundle of light, known as a photon. Atoms broadcast light only when they are in an excited state, and electrons that are above their normal position fall back from a higher step.

If all the elec¬

trons are on their normal steps, the atom does not send out any light. A beam of light from the sun or from a lamp contains millions of photons sent out by mil¬ lions of excited atoms. Hitting Atoms with Photons One way of pushing an electron to a higher step in an atom is to hit it with a photon. Then the energy of the photon is used up in raising the electron to the higher step. We say that the atom has absorbed the photon.

Usually, a photon will be absorbed only if

it has just the right amount of energy needed to raise the electron from a lower step to a higher step in the atom.

This amount is the same as the amount of

energy the atom loses when the electron falls from the

60

higher step to the lower step. An atom can absorb the same kind of photons that it can send out. Photons, as small bundles of energy, differ in the amount of energy that is in them. As waves of light, they also differ in the frequency of the waves. These two quantities, energy and frequency, are related to each other in a simple way. The more energy there is in a photon, the higher is the frequency of its wave, and vice versa. Since blue light has a higher frequency than red light, a photon of blue light has more energy than a photon of red light.

Ultra-violet rays have a

higher frequency than any visible light. So a photon of ultra-violet light has more energy than any photon of visible light. The Colors of an Atom The atoms in a gas are separated by spaces that are much larger than the atoms are.

Because of these

spaces, each atom in a gas behaves as if it were alone. When the atoms of a gas are excited, each one sends out a photon of colored light. The color of each photon depends on its frequency.

Its frequency depends on

its energy. The energy of the photon is the energy lost by an electron that fell from one level to another. Since every kind of atom has its own special set of levels, a gas made up of atoms of only one kind sends out only certain special colors of light. Because of this fact, you can identify atoms by the colors they send out when they are excited.

Chemists often do this by

61

>

means of a flame test.

To find out what metal is in

one of the chemicals they call salts, they put a small amount of the salt into a gas flame. The heat turns the salt into a gas and excites the atoms at the same time. If there is sodium in the salt, the flame turns yellow. Potassium turns, the flame violet.

Calcium, copper,

and indium make the flame brick red, green, and blue, in that order. An Atom’s Fingerprints If a beam of white light is passed through a gas, the atoms in the gas absorb some photons.

The atoms

absorb only the special colors that they can broadcast. So when the light emerges from the gas, it has less of these colors than others. It is as though the atoms had put their fingerprints on the light as it passed through. The atoms can be identified by the fingerprints they put on white light. To see the fingerprints, scientists take a narrow beam of the light and separate the colors in it to form a spec¬ trum.

The colors that have been absorbed by the

atoms are weak in the spectrum, so they show up as dark lines in the spectrum. In a liquid or solid the atoms and molecules are close enough to influence each other.

This increases

the number of levels on which electrons may stay. The atoms in a transparent liquid or solid also absorb photons from light that passes through.

Once again,

the absorbed colors are marked by dark lines in the

62

spectrum of the light that emerges. In the spectrum of a liquid or solid, there are so many of these lines that they bunch together to form bands. They are known as absorption bands. Molecules and radicals in a liquid or solid also put their fingerprints on light that passes through.

So they too form absorption bands in the

spectrum. There is a special instrument known as a spectro¬ photometer which not only separates the colors in light, but measures the amount of light of each color. These measurements, made on light that has passed through some material, can be used to identify the atoms or radicals or molecules that are in the material. The Color of Things The color of an object depends on its chemical nature. When white light shines on an opaque object, the light enters into the object for a short distance under the surface. On the way in, the photons in the light bombard the atoms and molecules that are there. Some of the photons are absorbed, and the rest are reflected back. sorbed.

Only certain special colors are ab¬

The light that is reflected is what is left of

the mixture we call white light after these colors have been removed. The color the object shows is the color of this reflected light. When white light shines on a transparent object, the light passes through the object. But, on the way through, some of the photons are ab¬ sorbed by the atoms and molecules they meet. So the

63

>

light that is transmitted is what is left after special colors have been removed. The color such an object shows is the color of the light that is transmitted. Chemists now know enough about the behavior of atoms, radicals and molecules to be able to predict what colors they absorb, and hence what colors they will show. In Chapter VII we shall see how this knowl¬ edge is used to make colored pigments for dyes, paints and inks. Color Made by Heat If a solid object is made hot enough, and it hasn’t melted or evaporated first, it begins to glow. We say it has become incandescent. This happens because the heat has excited the molecules in the object. Some of the heat is used up to raise electrons in the molecules to higher levels. Then, when the electrons fall to lower levels, they send out light, making the object glow. At first the electrons are not raised very far, so the photons they send out have a small amount of energy, or a low frequency.

So the color of the glow is red.

Then, as the object becomes hotter, the electrons are raised to higher and higher levels. Then, as they fall, they also send out photons with larger amounts of energy, or a higher frequency. As a result, the color of the glow changes as the object becomes hotter. When the light sent out includes all colors of the spectrum except blue and violet, the mixture of colors looks yellow.

When blue and violet are also included, all

64

colors of the spectrum are there, and the mixture looks white. That is why a heated object is first red-hot, then yellow-hot, and finally white-hot. In a tungsten lamp, the glowing wire is yellow-hot. We do not permit it to become white-hot, because then it would burn out too soon. Because the color and the temperature of an in¬ candescent object change together, we can use the color to measure the temperature.

Stars are incan¬

descent gases. They, too, can glow with different colors that depend on how hot they are.

So astronomers

measure the temperature of a star by means of its color. Cold Light There are some things that can be made to glow without being made hot. We can make them glow by shining light on them. A glow that is caused in this way is called fluorescence. The color of the glow in fluo¬ rescence is not the same as the color of the light that started it.

For example, blue light may make some¬

thing fluoresce with a green glow. The change in color is explained by what happens to the photons that are absorbed by the fluorescent material. The energy of each photon is divided into two parts. changed into heat.

One part is

The other part is broadcast as a

new photon. But the new photon has less energy than the old one, so its wave has a lower frequency and a different color.

Some fabrics are colored with fluo¬

rescent dyes so that they glow in the light.

65

>

There are many things that can be made to glow by shining ultra-violet rays on them.

The ultra-violet

rays are invisible, but the glow is made up of visible light.

Often a pigment that reflects one color when

ordinary light is shone on it glows with a different color when ultra-violet rays are shone on it. This fact is used in some advertising billboards to get two pic¬ tures out of one. The billboard is painted with fluo¬ rescent paints. The paints are arranged so that their ordinary colors form one picture, while their fluores¬ cent colors form another picture. An ordinary lamp and an ultra-violet lamp are shone on the billboard, one at a time. Each time one lamp is switched off and the other one is switched on, the picture that you see changes.

66

VI Nature’s Colors

Nature Uses Every Trick

I

n chapter

IV we described many ways of sepa¬

rating the colors that are mixed in sunlight. Nature uses all of them to produce the colors we see in the air, in the sea, and on the land. We know already that nature uses refraction, to pro¬ duce the colors of the rainbow. (See page 42). We have also seen that the scattering of light by particles in the air makes the blue of the sky and the red of sunset. (See page 46). Now we shall get acquainted with some more of the colors found in nature, and the ways in which they are produced. Baby Blue Eyes When babies are born, their eyes are blue. Then as the babies grow older, the eye color changes, some¬ times to green or different shades of brown. The colored part of the eye is called the iris. It is a

67

muscle made of loose threads. These threads produce the color of a newborn baby’s eyes. The threads scatter light in the same way that air does. So a baby’s eyes are blue for the same reason that the sky is blue. Later, as a baby grows older, a brown pigment be¬ gins to appear on the back part of the iris. This brown begins to show through between the threads.

The

brown and the baby blue combine to make the final color of the eyes. What this color is depends on how much pigment is formed, and how much of it shows through. The color of the sea is often described as blue. This color, too, is produced by the scattering of light. Small particles floating in the water scatter blue light just as particles in the air do.

Near shore, the blue color

mixes with the yellow color of sand to produce a green color. Mother of Pearl Many buttons are made of mother of pearl. This is the shiny inside part of some sea shells.

Mother of

pearl reflects patches of colored light that seem to move as you tilt it.

Mother of pearl produces colors

by diffraction, in the same way that a diffraction grating does. The shell is made up of many thin layers, and the surface we see cuts across these layers.

So the

surface has many thin grooves on it, like the scratches on a diffraction grating.

There is a simple way of

proving that the color of mother of pearl is produced by the lines on the surface and not inside the shell. If a

68

wax impression is made of the shell, the impression also has lines on it, because it has a ridge for every groove on the shell and a groove for every ridge on the shell. The wax impression then flashes the same irridescent colors that the shell does. A peacock’s tail makes a brilliant display of bright colors.

But if you look closely at a single feather of

the tail, you see that it is only a dull lifeless brown. The brilliant colors are produced by diffraction in the same way as the colors you see when you look through your eyelashes or through a piece of fine woven cloth. The wing cases of some beetles and the wings of some butterflies are covered with many parallel ridges. So they, too, produce colors by diffraction. The Fiery Opal The fiery opal is a precious stone made of many thin layers of equal thickness.

Light entering the opal is

partly reflected back at each surface between layers. The reflected beams behave like the beams reflected from the inner and outer surfaces of a soap bubble. Some colors, such as blue, are destroyed by the inter¬ ference with each other of waves that are out of step. Other colors, such as yellow and red, are strengthened. That is why the fiery opal has the color of a flame.

Green Grass and Red Blood Many of the colors in nature are produced by pig¬ ments that absorb some colors from sunlight and reflect

69

N

the rest. All plants contain the pigment chlorophyll, which plays an important part in the chemical process by which plants make their food. Chlorophyll absorbs nearly all of the red light that falls on it.

Plants use

the energy of the absorbed light to build sugar mole¬ cules out of water and carbon dioxide. The light re¬ flected by the chlorophyll in leaves is the light that is left over after the red light has been absorbed. This kind of light mixture looks green. That is why grow¬ ing plants are green. The red color of blood is caused by a pigment called hemoglobin.

In its chemical structure, haemoglobin

is a close relative of chlorophyll. In the blood of ani¬ mals, it does the special job of carrying oxygen to all parts of the body. Carrots and Flowers In the roots of carrots there is a yellow pigment called carotene.

Through slight changes in chemical

structure, nature has produced many forms of carotene. The carotenes are largely responsible for the yellows and reds that we see in the petals of flowers. The yel¬ low of daffodils and the yellow and red of marigolds, for example, are carotene colors.

The yellow of egg

yolk is also a carotene color. Carotene is the raw material from which vitamin A is made. It is changed into vitamin A by the action of ultra-violet rays.

70

Autumn Leaves At the end of the summer, when leaves begin to fall, the trees dress themselves in bright colors. The leaves change color from green to different shades of yellow and red. The yellow and red are caused by carotene in the leaves. During the spring and summer they are hidden by the green of the chlorophyll in the leaves. In the fall, the green fades as the chlorophyll disap¬ pears, and the yellows and reds come up out of hiding.

Apples, Skin and Hair If an apple is cut so that its flesh is exposed to the air, the surface of the cut begins to turn brown. The brown color is caused by a pigment called melanin that is formed by the action of the air on one of the chemicals that is in the apple. Melanin is also found in the skin and hair of human beings. We

saw

in

Chapter

II

that

sunlight

ultra-violet rays as well as visible light. rays are high-frequency waves.

includes

Ultra-violet

Because of the high

frequency, each photon of ultra-violet light has a high amount of energy. A photon with high energy is like a fast-moving bullet. It can damage the cells in a living body.

So the body needs protection against ultra¬

violet rays.

The melanin in our skins gives us this

protection by absorbing the ultra-violet rays before they can do any harm. In very sunny climates, where

71

\

people are exposed to a large amount of ultra-violet rays in

the sunshine,

they need more protection.

Nature has given them this protection by putting a large amount of melanin in their skins. That is why their skins are brown or black. Hair may have much or little melanin. The granules of melanin may be large or small.

Alongside the

melanin, there may be few or many air bubbles in the hair.

All these possible variations account for the

different colors that the hair of people may have. Lobster Red A change in the chemical structure of a pigment can change its color. We see an interesting example of this fact in the color of a lobster. A live lobster has a bluish color produced by a pigment in which carotene is combined with a protein.

When the lobster is

boiled, the carotene is separated from the protein. Then the carotene shows its own red color, and the lobster turns a bright red. The wild hyacinth is a blue flower. Its blue color is caused by a pigment called anthocyanin.

When an

acid acts on anthocyanin, it produces a slight change in its structure. This change in structure changes the color from blue to red. In England, children produce red hyacinths by planting the flowers near an ants’ nest. The ants produce formic acid, and the acid changes the color of the flowers from blue to red. 72

Color for Protection The color of many animals is not merely a decora¬ tion. It is a shield that protects them. It helps them get food to eat, or it saves them from being eaten. The coloring of animals takes three useful forms.

It may

be camouflage, which hides the animal. It may be a bluff, which frightens its enemies.

Or it may be a

disguise, which confuses its enemies or prey. Coloring serves as camouflage when it matches the background against which the animal rests.

For ex¬

ample, a tomato caterpillar is green, to match the leaves of the tomato plant on which it feeds. It looks so much like a leaf on the plant that it is not easily seen by birds that might eat it. A copperhead snake has patches of color that match the jumble of fallen leaves on the forest floor. So a copperhead resting on leaves is almost invisible. A good example of bluffing is given by the cater¬ pillar of a certain type of hawk moth. When the cater¬ pillar is at rest on the plant on which it feeds, it looks like a broken twig. But if it is disturbed, it turns over and puffs up until it looks like a snake about to strike. Black spots on its underside look like the eyes of the snake. An attacking bird, who thought he had found a caterpillar to eat, then finds himself in apparent danger of being eaten, so he turns and flies away. A fine example of protection by disguise is shown by the Viceroy butterfly, which looks almost like a 73

Monarch butterfly

Viceroy butterfly

Monarch butterfly. The Monarch has a bad taste that birds do not like, so after a bird has eaten one or two Monarchs, he tries to avoid them.

This saves many

Viceroys from being eaten, because the Viceroy looks so much like a Monarch. Colors We Use We use many of the colored pigments found in na¬ ture for making paints, inks, and dyes. The pigments may be divided roughly into two groups. One group consists of minerals. Usually a single atom or a radical is responsible for the color of a mineral pigment. The other group consists of pigments taken from plants or animals. These are called organic pigments. The molecules of organic pigments are far more compli¬ cated than those of mineral pigments. In Chapter VII we shall see how their colors depend on the structure of the molecules. Here are some of the common mineral pigments that are used: Iron oxide (Fe2Os) is a red pigment. It oc¬ curs mixed with sand in the earths called sienna or 74

(5 times actual size)

Purple came from the shell fish, Murex

Red came from the cochineal insect

ochre. Chromium oxide (Cr2Os) is a green pigment. It is used in the ink with which paper money is printed. A compound of cobalt, aluminum and oxy¬ gen makes up the pigment known as cobalt blue. Lead, chromium, and oxygen are joined together in chrome yellow.

Potassium, iron, carbon and nitrogen make

the pigment called Prussian blue.

Mercury and sul¬

phur combined give us the red pigment called ver¬ milion. Glass can be colored by adding to the glass small amounts of a particular atom. Here is a list of princi-

Indigo

Madder 75

>

pal colors, and some atoms that can produce them: red from copper; yellow from sulphur; green from iron; blue from cobalt; and purple from nickel. Organic colors are valuable for dyeing cloth.

In

ancient Rome, imperial purple was obtained from the shell fish Mnrex. In the British isles, at the same time, the natives stained their skins blue with a dye made from the leaves of the plant called woad.

The dye

called indigo originally came from the roots of a plant that grows in Asia. Alizarine, a red dye, came from the roots of the madder plant.

Carmine, another red

dye, was obtained from the cochineal insect that lives in Mexico. However, we no longer depend on plants and animals to make the organic dyes we use.

We

shall see in the next chapter that we have learned how to make them ourselves.

76

VII Improving on Nature

More and Better Dyes

W

e color fabrics with dyes made of organic

pigments. They are called organic because the first ones used came from living organisms, that is, from plants and animals. But now we no longer rely on plants and animals to make the pigments for us. We make them ourselves in chemical laboratories.

We

put together, or synthesize, all the old dyes found in nature, and many new ones not found in nature at all. There were only a few natural dyes that were used for many hundreds of years. The color they gave to fabrics sometimes did not last long, because it was soon washed out or faded. Now we have thousands of syn¬ thetic dyes. They give us more colors to choose from. They also last longer than the natural dyes did. The chemist has not only imitated nature.

He has im¬

proved on it. There was a time when chemists thought that or¬ ganic chemicals could be made only by plants and ani77

mals.

This belief was proved wrong in 1828 when

Friedrich Wohler made the compound urea in his laboratory. After that, chemists learned how to make other organic chemicals, including some that had never been seen before. The first organic dye, mauvine, was made by William Henry Perkin, in 1856, when he was only seventeen years old. Carbon Compounds To see how organic pigments are made, first we must get acquainted with some facts about organic chemis¬ try. All organic chemicals are compounds of carbon. We saw in Chapter V that an atom of carbon has six electrons surrounding its nucleus.

Two of them are

in the first shell. The other four are in the second shell.

Valence bond

H H

Carbon (C) has four electrons to share, so it can join with four hydrogen (H) atoms to make methane

This shell has room for eight electrons, so it is incom¬ plete. Atoms that have incomplete outer shells form partnerships with each other to share their outer elec¬ trons.

When two atoms are partners, they put to¬

gether equal amounts of electrons to form pairs, and then they share the pairs. Carbon has four electrons to share, so it can join with as many as four other atoms at a time. A pair of shared electrons is called a valence 78

Simplified diagrams

H

The benzene ring

Benzene

Nitrobenzene

bond. In diagrams of chemical compounds, a valence bond is shown as a line joining the atoms that share the electrons.

In these diagrams, each carbon atom

is surrounded by four lines, because each carbon atom forms four bonds to neighboring atoms. The Benzene Ring Sometimes many carbon atoms are joined to each other in a long chain, like a line of people holding hands. If the carbon atoms at the end of the chain are joined to each other, too, the atoms form a ring. The molecule of benzene is an example of such a ring. There are six carbon atoms in a benzene ring. Each

Naphthalene

Anthracene 79

carbon atom in the ring is joined to two other carbon atoms and to one hydrogen atom.

In the usual dia¬

gram for the benzene ring, the six carbon atoms are shown forming a hexagon, or six-sided figure.

For

this reason a hexagon with no letters printed around it is sometimes used as a simplified diagram for a ben¬ zene ring. In the simplified diagram it is understood that there is a carbon atom at each corner of the hexa¬ gon, and that a hydrogen atom is joined to each corner. If the molecule is changed by putting a group of atoms in the place of a hydrogen atom, the symbol for the group is printed at that corner.

The third diagram

on page 79 represents nitrobenzene, which is formed by putting N02 (a nitrogen atom joined to two oxygen atoms) in the place of a hydrogen atom at one corner of a benzene ring. Many complicated molecules are made up of several benzene rings joined together. Napthalene consists of two benzene rings. Anthracene is made up of three benzene rings. Ring molecules like these are the raw material out of which colored pigments are made. Deepening of Color Benzene is a colorless compound. But colored com¬ pounds can be made from it by a process that the chemists call color deepening. hind the processes.

Here is the secret be¬

When sunlight passes through

benzene, bands of ultra-violet light are absorbed.

If

the wave lengths of the absorbed rays were just a little 80

longer, they would be rays of colored light. They can be made a little longer by changing the molecules of benzene

to

make

them

heavier

and

larger.

The

changed, heavier molecules form a new compound which is colored because it absorbs some colored light. The diagram below shows how different colors are obtained by making the wave lengths of the absorbed rays longer and longer. Seven spectra are shown, one under the other. The center section of each represents wave lengths of visible colored light, from red to violet. The section on the left represents infra-red light. The section on the right represents ultra-violet light. The dark shading in each spectrum shows wave lengths With this spectrum... Hi

_ '


H

H

I

HO

H

OH

H

H

H

Indigo has been changed to become soluble

A Treasury of Color Colored pigments are made from compounds like benzene, napthalene, and anthracene, whose mole¬ cules are built out of rings. To make many different kinds of pigments, the chemist has to have a large supply of such molecules.

He finds them in a great

treasure house in which nature has stored them. This treasure house is ordinary coal. Coal contains a sticky liquid called coal tar.

When coal is baked to make

coke, the coal tar is boiled out of it.

The coal tar

contains the ring molecules that the chemist can use for making dyes.

Coal tar has many other uses, too.

By making the proper changes, the ring molecules in coal tar are turned into such things as drugs, perfumes, and explosives, as well as thousands of different dyes.

86

VIII The Color We See

From Light Source to Brain

^17"hen we

’ "

see the color of an object, it is the

last step in a chain of events that begins

at a light source. The light source sends out a mixture of light rays of many wave lengths. As the rays pass through the air to the object, the air removes some of the light of short wave length by scattering it. When the light falls on the object, the object removes some more light by absorbing it. What is left of the light is then reflected or transmitted through the air to our eyes.

Once again the air removes some light.

The

color we see depends on the kind of light mixture that finally reaches our eyes.

But it depends, too, on the

nature of our eyes, and the messags that our eyes send to the brain. Camera, Photocells, Cable and Computer The eye is like a camera. In a camera, light enters 87

a box through a small opening and passes through a lens.

The lens focuses the light onto a sheet of film

that is opposite the lens.

In the eye, light enters

through the pupil, an opening in the iris, and passes through a lens. The lens focuses the light onto a screen called the retina, which lies opposite the lens. In the retina there are small cells that are sensitive to light. These cells behave like photoelectric cells. When light falls on them, they produce an electric current.

In

each retina there are about seven million of these cells lying side by side.

Each cell is connected by a nerve

to the brain. The nerve is like a telephone line from the cell to the brain. The electric current produced

The eye by the cell is a message that, travels over this line from the cell to the brain. The nerves from all the cells in one eye come together behind the retina in the optic nerve which runs to the brain. The optic nerve is like a cable in which the many wires that carry messages from the eye to the brain lie side by side. The brain 88

is like an electronic computer. It receives the messages and organizes them into a picture which it interprets. The color that we see is part of the picture that is or¬ ganized by the brain. Cones and Rods There are two kinds of cells for receiving light in the retina. They are called rods and cones. Each kind of cell does a different job for us. The rods are sensi¬ tive to different amounts of brightness. They let the brain know whether the light they receive is bright or dim. The cones are sensitive to different wave lengths, or colors. They let the brain know whether the light they receive is red, orange, yellow, green, blue, or violet. The rods and cones are not distributed evenly over the retina. In the center of the retina there is a small area called the fovea where there are only cones. This is the part of the retina which sees things best. When we want to see something clearly, we look straight at it, so that the light from it falls on the fovea in the retina of each eye.

Outside the fovea, there are both rods

and cones. The number of cones is smaller at greater

Rods

Cones

► 89

distances from the fovea.

At the outer edge of the

retina, there are rods but no cones at all. The edge of the retina cannot see color. But it is very sensitive to even small amounts of brightness.

That is why,

when you look at something very dim, like a faint star in the sky, you see it best by looking at it “out of the corner of your eye.” Although each cone and rod is tiny, it still takes up a definite amount of space in the retina. This puts a limit on how much detail we can see. If two spots that we look at on an object are very close to each other, the light rays from these spots fall on points of the retina that are close to each other. If the two spots are close enough to each other, the light rays from both spots will fall on one and the same cone or rod. In that case the two spots look like only one spot to the eye. When Gray Is White or Black There are some reflecting surfaces that scatter the light they reflect, but treat all colors equally.

Either

they reflect all the light of all colors, or none of the light of all colors, or the same fraction of the light of all colors. In Chapter III we said such a surface looks white, black or gray, depending on whether it reflects all, none, or part of the light. This statement is not entirely true.

There are times when a gray surface

looks white or black. To check this last statement, cut a disk out of black paper. Hang the disk up in a room that can be made 90

completely dark at night by drawing the blinds. Shine a flashlight on the disk and turn off the room lights. When your eyes are used to the darkness of the room, the disk, lit up by the flashlight, will look white. This shows two things: First, the disk does reflect some light, so it is really a gray disk. Secondly, a gray disk can look either black or white as well as gray. The secret be¬ hind this paradox is this: A gray object looks white when it is the brightest thing of all the things you see at the same time.

It looks black when it is the least

bright thing of all the things you see at the same time. It looks gray when there are brighter objects and less bright objects that you can see at the same time. So whether an object looks white or black or gray does not depend only on the light that it sends to your eyes. It depends also on a comparison between this light and the light sent to your eyes by other objects. This com¬ parison is made by your brain. Matching Colors When light comes to your eyes through a yellow filter, you see it as yellow light.

But if you pass this

light through a prism to form a spectrum, you see that it is really a mixture of many colors, usually red, yellow and green. On the other hand, the yellow light in the spectrum cannot be separated into other colors. If you pass a beam of it through a prism, only yellow light comes out on the other side of the prism. In one case, light that looks yellow may include light whose wave 91

lengths are as high as 700 m/i (red light of the spec¬ trum) and as low as 500 m/* (green light of the spec¬ trum).

In the other case, light that looks yellow in¬

cludes only light whose wave length is from 570 to 590 m/* (yellow light of the spectrum). So we see that two samples of light may look alike even though they are different mixtures of colors of the spectrum. We say that two mixtures of light match each other if they look alike to the eye. We can mix colors of the spectrum in different amounts by flashing them from different projectors onto one spot on a white screen. The screen will mix the colors because it reflects them all to our eyes at the same time. We can compare two mixtures by projecting them to spots in the screen that are side by side. By changing the amounts of spec¬ tral colors in the mixtures we can find many different mixtures that match. Matching Gray There are many mixtures of light that look white or gray. Some of them, like those reflected from a gray surface bathed in sunlight, include all the colors of the spectrum.

There are others that include only two

colors. Two colors which produce white or gray when light of these colors is mixed are called complementary colors, and each is called the complement of the other. Pairs of complementary spectral colors are shown in the following table. 92

Complementary Spectral Colors Red.Blue-green Orange-red.Green-blue Orange .Blue Yellow.Blue-violet Green light mixed with purple light also produces white or gray.

This pair has not been put into the

table because purple is not a color of the spectrum. Purple light is a mixture of red light and blue light. So a mixture of green light and purple light is rfklly a mixture of three spectral colors. The Color Circle In the spectrum of sunlight, the spectral colors are arranged in a line, from red to violet. We can form another line of colors that range from red to violet by mixing different amounts of red and violet light. The eye sees each such mixture as a kind of purple.

A

purple close to the red end of the line has more red light in it than violet light, and is called a red-purple. A purple close to the violet end of the line has more violet light in it than red light, and may be called a violet-purple. The line of spectral colors and the line of purples may be joined to form a circle, as shown in the diagram on page 94. All the colors that appear in this circle are 93

known as hues. The circle of hues has three kinds of information stored in it.

First, it shows the spectral

colors arranged according to wave length.

Secondly,

it is a kind of color-mixing chart. If wTe mix light of two hues that are close to each other on the circle, the mixture matches a hue that is between them on the circle. For example, yellow light mixed with red light matches orange light. Thirdly, it is a chart of comple¬ mentary pairs. Each hue is the complement of the hue that lies on the opposite side of the circle. Three Hues Make All The hues red, blue and green have a very important property. If we mix the right amount of light of these three hues, we can match closely any hue on the color circle. For this reason, red, blue and green are called

The color circle

94

primary colors.

There are other sets of three hues

that have the same property, but the red-blue-green set is the one that is used most often. The Color Tree If we want to describe a color, first we look for a hue on the color circle that looks most like that color. If we compare two colors that have the same hue, we find that they may still look different. One may seem to be brighter than the other. Or, even if they both look equally bright, one may seem to be more strongly White /'"N

The color of this branch...

)

)

c °ircle A color at this end of the branch . . .

has less

The color tree 95

Branches that lie above each other have colors of the same hue

colored than the other.

In that case, we say that the

more strongly colored one has more chroma, example opposite.)

(See the

To describe a color completely

we must say what its hue is, how bright it is, and how much chroma it has. The circle of hues is not a complete chart of colors. It is extended into a complete chart of colors in the Munsell color tree. Every color is on a branch of this tree.

Branches that lie over each other have colors

with the same hue. The different hues are arranged around the trunk of the tree as they are in the circle of hues. The height of a branch shows the brightness of the colors that are on it. The higher the branch is, the brighter are its colors.

The distance of a color

from the trunk shows its chroma.

The farther the

color is from the trunk, the more chroma it has. Color Mixing Two different ways of mixing colors are shown in the next drawings.

The first drawing shows what

happens when a blue spotlight and a yellow spotlight shine on the same white surface. The blue light and the yellow light are added to each other on this sur¬ face, and the mixture looks white.

Here blue and

yellow were mixed to produce white.

This kind of

mixture is called additive, because it is made by adding one kind of light to another. The second drawing shows what happens when a blue filter and a yellow filter are placed one after the

The spectrum of sunlight (See page 93)

These two colors differ chiefly in brightness (See page 95)

These two colors differ chiefly in hue (See page 94)

These two colors differ chiefly in chroma (See page 96)

ibtractive mixing of colors (See page 98)

Additive mixing of colors lSee baae 96)

After image (See page 105) Stare at the orange square for about 20 seconds. Then look at the black dot.

How background affects color (See page 105) The color you see where the ink band is printed over white is mostly the color transmitted by the ink. The color you see where the ink band is printed over black is the color re¬ flected by the ink. (See page 113)

\

Blue

Additive mixture of colors other in the path of a beam of white light. The blue filter removes or subtracts from the white light all of the red and yellow light that is in it. So the light that passes on from the blue filter is a mixture of violet light, blue light, and green light. This mixture looks blue.

The yellow filter subtracts from this mixture

all the violet light and blue light that is in it. So the light that passes on from the yellow filter is green. Here blue and yellow were mixed to produce green.

White

Subtractive mixture of colors

97

This kind of mixture is called subtractive, because it is made by removing or subtracting colors from white light in two steps. When a painter mixes two colors on his palette, he is using subtractive mixture.

The pigment in each

color is like a filter that removes some colors from white light. Mixing the pigments is like placing one filter after the other in the path of the light that strikes the paint. Mixing Colors in the Eye Make two dots very close to each other on a piece of paper, and then move the paper away from your eyes. When the paper is far enough away, you will not be able to see the dots as two separate dots. They will merge into one single dot.

When this happens, the

light from the two dots falls on the same spot on the retina of your eye and is mixed there. If the two dots have different colors, you see both colors as long as you see the dots as two separate dots.

But when the

dots are so far away that they seem to merge into one dot, the two colors mix on your retina. Then you see only one color, produced by additive mixture of the two colors. Many of the colors that you see outdoors are pro¬ duced in this way by mixing colors in your eyes. In the fall, when you stand close enough to a tree to see the separate leaves on its branches, you may see greens and reds side by side on the same tree. But if you are far enough away, you cannot see the leaves separately, and

98

their colors mix in your eyes.

So, at a distance, the

tree looks brown, which is the color produced by additive mixture of red and green. There are painters who mix their colors in your eyes instead of mixing them on their palettes. They make a painting out of small points of color placed very close to each other on the canvas. When you look at the painting from a distance, the points of color merge and the colors mix in your eyes. This method of painting is called pointillism. It was used by some of the French painters of the Impressionist school. Painters who use pointillism must follow the rules for additive mixture of colors, while painters who mix colors on a palette use the rules for subtractive mixture. Seeing What Isn’t There Suppose a lamp is turned on and off quickly, so that the lamp glows brightly for a short time, and then becomes dark. When the light from the lamp enters your eye and falls on the retina, you see the glow of the lamp. But you do not see it at once. There is a slight delay before the cells in your retina notice the glow. When the lamp grows dark again, there is an¬ other delay before the cells in your retina notice that the glow is gone. They continue to see the glow for a short time after there is no glow to see. This strange behavior of the retina is known as persistence of vision. In a motion picture theatre, a series of pictures is flashed on the screen, one picture after another.

Be¬

cause of persistence of vision, each picture seems joined

99

to the next one. So, instead of seeing a series of still pictures on the screen, you see one picture that seems to move. Persistence of vision gives us another way of pro¬ ducing an additive mixture of colors in our eyes.

If

we flash two colors to our eyes, one after the other, in rapid succession, we begin to see the second color be¬ fore the first one dies out. As a result we see the two colors mixed.

A simple device for mixing colors in

this way is the Maxwell top. making one.

Here are directions for

First get these materials: a

(round-

headed) machine screw one and one-half inches long 4 inches

4 inches

Cardboard base

Colored disk

You spin the top by turning the screw

The finished top The Maxwell top

100

and one-eighth of an inch wide, with a nut to fit; a sheet of stiff cardboard;

and colored construction

paper in a variety of colors. Cut out of the cardboard a circular disk four inches in diameter. Make a oneeighth-inch hole in the center of the disk, and push the shaft of the screw through the hole until the head of the screw touches the disk. This makes the base of your top.

Now cut four-inch disks out of the con¬

struction paper. Make a one-eighth-inch hole through the center of each disk, and cut a slit in each disk from the center to the rim. To mix two colors, say red and yellow, put a red disk on the shaft of the top and put a yellow disk over the red disk, so that the two slits are over each other. Separate the edges of the red disk at the slit and slide one end of the yellow disk into the slit, so that half of the yellow disk is hidden under the red disk.

Then

fasten the disks in place by putting the nut over the shaft and tightening it. The upper face of your top is now half red and half yellow.

To mix the colors,

simply spin the top. By sliding the disks on the top, you can change the proportions of the two colors on the top. You can also put more than two colors on the top, using each of the lower disks to hide part of the disk that is over it. Verify by means of a Maxwell top that red, green, and blue, mixed in the right proportions, produce gray. Color from Black and White The disk shown on page 102 is called a Benham disk.

101

The Benham disk

Although it is only black and white, you can use it to produce colored rings. To make a disk like this one, first cut a four-inch disk out of white paper. Use india ink to paint half of the disk black. On the white half of the disk, draw black arcs three-sixteenths of an inch wide and one-eighth of a circle long, in two sets of three each, as shown in the picture. Place the disk on the base of your Maxwell top and spin it. When you spin it, the arcs on the disk seem to close up to form six rings. If the speed is not too high, the rings will be colored. When the top spins clockwise, the outer rings are blue and the inner rings are red.

If you

spin the top the other way, the colors change places.

102

The reason why these colors appear on a black and white disk is not fully understood.

However, it is

probably related to persistence of vision, and the fact that the different colors in white light persist for un¬ equal lengths of time. Getting Used to the Dark When you go outdoors at night from a brightly lit room, at first you have trouble seeing in the dark. But then your eyes get used to the dark, and you begin to see distinctly things that you could not see before. Your retina actually becomes more sensitive to light, so that you can see things that are very dim. When you return to the brightly lit room, you have trouble seeing in the light, because everything seems too bright. But after a while, the brightness seems to fade. Your retina actually becomes less sensitive to light, so that you see well only things that are bright. The light of a flashlight, that looked bright outside, seems feeble when you turn it on in a well-lit room. The change in sensitivity of your retina which makes your eye get used to seeing in the light or in the dark is called adaptation. The adaptation of your eyes to light or darkness ex¬ plains why, when you drive a car, you do not turn on any lights inside the car. If you had a light on inside the car, your eyes would become less sensitive to dim light. Then you would have trouble seeing things in the dark roadway.

103

Changes We See and Do Not Notice As the sun rises higher during the day, more and more light pours into the window of your room.

A

white sheet of paper in your room, bathed in this light, reflects more and more light to your eye.

But

you hardly notice this fact at all. The brightness of the sheet looks about the same to you all day. At night, when you see this sheet by the feeble light of a lamp, the sheet reflects very little light to your eyes. But it still looks to you like the same bright white sheet that you saw in the daytime. The brightness of a thing does not seem to change much even though the light in which we see it changes. A white sheet of paper reflects all the light that strikes it.

In the daytime you see the paper by day¬

light, which comes from the blue sky. Then the light reflected by the paper is largely blue.

But the paper

does not look blue. It looks white. At night, you see the paper by lamplight, sent out by a yellow-hot wire in your electric light bulb.

Then the light reflected

by the paper is largely yellow. But the paper does not look yellow.

It looks white.

Near a brick wall, the

paper reflects red light which it receives from the red bricks.

But the paper does not look red.

It looks

white.

Under a tree, the paper is bathed in green

light reflected by the leaves of the tree. But the paper does not look green. It still looks white. Even though the color of the light that comes from the paper changes, the color you see in the paper stays the same.

104

These facts show that the brightness and color you see do not depend only on the light that is sent to your eyes.

They depend also on how sensitive your eyes

are, and on how your brain interprets what your eyes see. One Color Makes Another On the sheet of colored pictures following page 96, there is an orange square. Near the square is a black dot printed on a gray background. Stare at the square for about twenty seconds, and then shift your eyes to the black dot. After a short time, you will see a blue square there. The blue square is not on the page. It is an image in your eye caused by looking at a gray surface after you have stared at an orange square. We call it an afterunage. If the square were red, the after¬ image would be blue-green. The afterimage is always the complement of the color you stared at. Stare at the orange square again, and you may notice traces of blue just outside the square. When you look at any colored object, the things that are right next to it begin to show a bit of the complementary color. For this reason, the color you see in an object ahvays depends in part on the color that surrounds it. This is shown very clearly in the pictures following page 96. There are three sets of four squares. In each set the four squares have exactly the same color but are printed against different backgrounds. Because of the different backgrounds, the colors of the squares look different, even though they are the same.

105

How We See Color Scientists have spent many years trying to find out how the eye sees color.

Many different theories have

been offered. One of them has finally been proved to be true. This theory, first proposed in 1801 by Thomas Young, was developed in detail by Hermann Helmholtz. It was popular even before it was proved because it was useful. According to their theory, there are three dif¬ ferent kinds of cones in the retina. One kind is very sensitive to red light. A second kind is very sensitive to blue light. The third kind is very sensitive to green light. The theory assumes that red light stimulates only the “red” cones, blue light stimulates orily the “blue” cones, and green light stimulates the “green” cones, but also stimulates the others just a little. Other colors were assumed to stimulate all three types of cones in differ¬ ing amounts. The Young-Helmholtz theory was finally proved in 1964, when George Wald and Paul K. Brown showed that there really are three different kinds of cones, one kind most sensitive to red light, a second kind most sen¬ sitive to blue light, and the third kind most sensitive to green light. There is one important catch about the YoungHelmholtz theory.

There is no evidence that there

really are three different kinds of cones in the retina. The best we can say is that the retina behaves as if there were three kinds of cones.

106

New Discoveries Recent discoveries made by the American scientist Edwin H. Land show that the way in which the eye sees color is far more complicated than Young and Helmholtz thought. Land showed that he could make the eye see all colors even when it receives only two colors of light. He took two photographs of a colored scene. One photograph was made with long waves of light coming from the scene, transmitted through a red filter. light. screen.

The other was made with short waves of

Then he projected the two pictures onto a He used light with a long wave length to

project the picture taken with long waves.

He used

light with a short wave length to project the picture taken with short waves. On the screen, where the long and short waves mixed, they produced a picture show¬ ing all the colors of the original scene. Land’s experi¬ ments showed that the color the eye sees on any part of the retina depends on how much long-wave and short-wave light reaches that part of the retina.

But

it also depends on how much of each reaches other parts of the retina at the same time. In other words, when we see color in a scene, we are comparing the light that comes from one part of the scene with the light that comes from every other part of the scene. This comparison is made by the brain. So a complete theory of how we see color will have to explain how the brain interprets the messages it receives from the eye.

107

Color Blindness Most people will agree on whether or not two colors match.

But about 3 per cent of the population will

disagree with the rest.

These people, mostly men,

see colors in a different way. Often, where most of us see two distinct colors, they see only one color. We say that these people are color-blind. For most people, red, blue and green serve as pri¬ mary colors. By mixing the right amounts of light of these three colors, they find that they can match any color.

People with the most common form of color

blindness have only two primary colors.

By mixing

light of these two colors in the right amounts, they find that they can match any color.

Of course, what

looks like a match to them does not look like a match to everybody else. We cannot say that color-blind people match colors incorrectly.

We can only say that they match colors

differently. In fact, we say they are color-blind only because they are in a minority. If the percentages were turned around, so that they became 97 per cent of the population, and the rest of us were only 3 per cent, then we would be the ones who are color-blind! This fact emphasizes in a striking way that the color we see does not depend only on the thing we look at and the light by which we see it. It depends as well on the way our eyes receive the light, and how our minds interpret what the eyes receive.

108

IX Color in Printing, Television, and Shading with Dots

J

\\ hen you draw a picture, you can put shading

* *

into the picture by means of dots.

For a

light shadow you use small dots separated by wide, spaces.

For a dark shadow you use large dots, and

crowd them together. Newspapers and magazines use the same idea when they print black and white pictures with shading. They print the picture as a collection of dots. The dots are large where the picture is sup¬ posed to be dark. They are small where the picture is supposed to be light. You usually do not notice the separate dots when you look at the picture, because the black dots and the white spots between them mix in your eyes to produce shades of gray.

But if you

look at the picture through a magnifying glass, you can see the separate dots clearly.

Shaded pictures

printed by means of dots are called halftones.

109

Dots Made One at a Time A picture on a television screen is something like a halftone print. It is made up of many light and dark spots on the face of the screen.

But it differs from a

printed halftone in several interesting ways.

First,

while a halftone print is made by putting dark spots on a light background, a television picture is made the other way around. The picture is formed by mak¬ ing light spots on a dark background. The face of the screen is covered with a phosphor, a chemical that glows when it is struck by electrons. An electron gun behind the screen fires a stream of electrons at it. A light spot is formed where the electrons strike the screen and make it glow. A dark spot is a part of the screen that is not glowing.

Secondly, in a halftone

print, the dots are all there at the same time, side by side. But in a television picture, the glowing dots are made one at a time, one after the other. The electron gun scans the screen, the tvay your eye scans a printed page when you read. It forms the glowing spots on the screen in lines, with many lines one under the other. Although the spots are made one at a time, they are seen together side by side for two reasons. First, when a spot is struck by electrons, it glows for a while. Secondly, because of persistence of vision, the eye con¬ tinues to see the glow for a short time after it has died out. So, while the glowing dots are made one at a time, many are seen at the same time. When a scene is broadcast for television, a television camera scans the scene.

The light and dark spots of

110

the scene send varying amounts of light to the camera. The camera converts the light from each spot into an electrical signal. A bright spot becomes a strong signal. A dark spot becomes a weak signal. These signals are sent out, one after the other, to the television receiver. In the television picture tube, the electron gun scans the screen while the camera at the broadcasting station scans the scene.

A strong signal produces a strong

flow of electrons and makes the spot that it strikes glow brightly.

A weak signal produces a weak flow

of electrons, and a feeble glow. In this way, light and dark spots are produced on the screen that match the light and dark spots of the scene that is being broad¬ cast. Color Television To produce a colored picture on a television screen it is not enough to put light and dark spots on the screen.

It is necessary to have colored spots on the

screen. This is made possible by using phosphors that glow with colored light. A colored picture may have dozens of different colors in it. If each color had to be produced with a different phosphor, making colored pictures on a television screen would be far too complicated and expensive. Fortunately, this is not necessary.

All hues can be

matched by mixing the right amounts of the primary colors, red, green, and blue. So, to make a picture in full color, all you need is a practical way of mixing red, green and blue in different amounts for different

111

parts of the picture.

A practical way of mixing the

colors is the method of pointillism used in painting. If the colors to be mixed are produced as small dots side by side, then the colors are mixed in the eye that looks at the picture. The only problem left is to find out where to put the dots. This problem is solved in color television by making three halftone pictures, one in red, one in green, one in blue. The three halftone pictures are produced on the same screen so that red, green and blue dots occur side by side. Here are the main steps for making and combining the three colored halftones: A scene to be broadcast in color is reflected from three special mirrors into three separate television cameras.

Each of the mirrors re¬

flects only one of the primary colors, so that each camera receives a different picture.

In each camera,

the spots of light it receives are converted into elctrical signals. A rotating switch picks up the signals from each camera in turn, over and over again. In this way, the three chains of signals are merged into a single chain of signals.

This chain of signals is broadcast

to the receiver. In the color television receiver, a special kind of pic¬ ture tube is used. The screen of this tube is covered with spots of three different phosphors. When struck by electrons, one kind glows red, the second kind glows green, and the third kind glows blue. The spots are arranged in triples over the face of the tube, with one spot of each kind in each triple. There are three elec¬ tron guns behind the screen. One scans the spots that glow red. A second scans the spots that glow green. The

112

third scans the spots that glow blue. A rotating switch connects the incoming signals to the three guns in turn. In this way, the three chains of signals that were merged at the broadcasting stations are separated again. Each chain of signals controls one electron gun, and produces a picture in one color on the screen. The three colored pictures combine to form a colored copy of the original scene. Color Printing Many colored pictures are printed in magazines, books, and newspapers. In the most common method of color printing, three colored halftones are printed on the same sheet of paper. The inks that are used are transparent. This fact makes it possible to mix colors on the paper in two ways. The colors can be mixed additively by printing colored dots side by side.

But

they can also be mixed subtractively by printing colored dots one over the other.

To take advantage

of the subtractive mixing, printers do not use the primary colors, red, green and blue. complements instead.

They use their

The complement of red is a

blue-green called cyan. The complement of green is a red-purple called magenta. The complement of blue is yellow.

These three colors are sometimes called

subtractive primaries, while red, green and blue are called additive primaries. They are the colors of the inks used in color printing. In addition a black ink is used to provide shading in the picture. The complete picture is made by printing four halftones on the same

113

sheet, one of them in black and the other three in color. Each of the colored inks, when it is printed on white paper, acts as a filter for the light reflected through the ink by the paper.

To describe the be¬

havior of these filters it is convenient to think of the spectrum of white light as being made up of three parts, a red part

(long waves), a blue part

waves), and a green part (middle-sized waves).

(short The

cyan ink removes the red light reflected by the paper and transmits the blue and green. The magenta ink removes the green light and transmits the red and blue. The yellow ink removes the blue light and trans¬ mits the red and green. If two of these inks are printed on top of each other, one of the additive primary colors is produced. If cyan and magenta are printed on top of each other, red and green are removed from the reflected light and only blue light is transmitted. If cyan and yellow are printed on top of each other, red and blue are removed from the reflected light, and only green is transmitted. If magenta and yellow are printed on top of each other, green and blue are re¬ moved and only red is transmitted.

So using cyan,

magenta and yellow inks, the printer produces red, green and blue by subtractive mixture.

He then has

six colors to work with as well as black and white. He mixes these additively to produce more colors when he prints them as small dots side by side, in halftones printed on one sheet. Color Photography There are many different methods of color photog-

114

raphy in use. We shall describe briefly only the Kodachrome process, which produces colored transparen¬ cies that you can project onto a screen. In the Kodachrome process, three pictures are made one behind the other in a single sheet of film.

Each

picture contains a single color. Each picture acts as a filter for the light that passes through it when the pic¬ ture is projected onto a screen.

Since each filter re¬

moves part of the light that passes through it, the colors are mixed according to the rules of subtractive mixture. So the Kodachrome process, like color print¬ ing,

uses

the

subtractive

primary

colors

yellow,

magenta and cyan. Negatives and Positives In ordinary black and white photography, you take a picture by allowing light to fall on a sheet of photo¬ graphic film.

The film is coated with an emulsion

made of a kind of silver compound called a silver halide.

Wherever light strikes the halide, it is “ex¬

posed.” Then the film is developed by treating it with special chemicals.

In the process of developing, the

exposed part of the halide is converted into silver, so that it turns black. The unexposed part remains clear. The result is a negative, in which the light and dark of the picture are reversed.

The negative is black

where the picture should be white, and vice versa. To make a positive print of the picture, you usually send light through the negative to expose a separate print paper. Then when you develop the print paper, you have a negative of the negative.

115

There is another way in which a positive print might be made, in the same film that contains the negative. In the negative, the part of the halide that was exposed has become black. If this part could be washed out, it would become clear.

Then, if the remaining un¬

exposed halide is exposed and then developed, what was clear in the negative would become black. In this way the negative can be changed into a positive. This is actually done in the Kodachrome process. Now let us trace the main steps in the Kodachrome process, from taking the picture to making the trans¬ parency. Emulsion sensitive to blue light Yol low filter' Emulsion sensitive to blue and green lie light Emulsion sensitive to blue and red lighl

Cross-section of Kodachrome film

Three Films in One The film used in the Kodachrome process is really made of three films in one, so that it takes three pic¬ tures at the same time. There are three emulsions in the film, one on top of the other.

The drawing

above shows the arrangement of the emulsions.

The

first emulsion is sensitive only to blue light. The sec¬ ond emulsion is sensitive to both blue and green light, but not to red light. The third emulsion is sensitive to blue and red light, but not to green light. There is a yellow filter between the first and second emulsion.

116

Taking the Picture When we take a picture, a mixture of light is sent through the film. It may be thought of as a mixture of red, green and blue light.

The first emulsion is

affected only by the blue light in the mixture, if there is any. The yellow filter between the first and second emulsion then removes the blue light, so that only green light and red light pass on to the second and third emulsions. The second emulsion is affected only by the green light, and the third emulsion is affected only by the red light. The result is that three separate pictures have been taken at the same time, using blue, green, or red light only. The second diagram on page 119 shows the effect on the emulsions of taking a pic¬ ture of something that is colored only blue or green or red. Making the Picture The first step in producing a color picture from the exposed film is to develop it the way an ordinary black and white picture is developed. In each emulsion, the development makes the exposed parts of the halide turn black.

Each of the three emulsions is now a

negative. Now the film is exposed to blue light.

The blue

light exposes what is left of the silver halide in the first emulsion.

The film is developed a second time

in a special solution that converts the silver halide into silver, and attaches a yellow dye to it at the same time.

117

In this way a yellow-colored positive is made in the first emulsion. Only that part of the emulsion is dyed that was not exposed when the picture was taken. Now the film is exposed to green light.

This ex¬

poses what is left of the silver halide in the second emulsion.

Then the film is developed again.

This

time, as the silver halide is changed into silver, a ma¬ genta dye is attached to it. So a magenta-colored posi¬ tive is made in the second emulsion. Then, the film is exposed to red light. This exposes what is left of the silver halide in the third emulsion. Then the film is developed again. This time, as the silver halide is changed into silver, a cyan dye is at¬ tached to it. So a cyan-colored positive is made in the third emulsion. The third diagram on page 119 shows the emulsions after they have been dyed. Finally, the silver deposits in all three emulsions, and the yellow filter between the first and second emulsions are washed out.

The transparency is now

complete. In each of the three layers, the film is clear where it was exposed when the picture was taken. It is colored everywhere else.

The fourth diagram on

page 119 shows a cross-section of the completed trans¬ parency. The fifth diagram shows what colors you see when you look through the transparency. There is one peculiar fact about this complicated process that you have surely noticed. The emulsions were exposed by blue, green, and red light, in that order. But they were dyed yellow, magenta, and cyan, which are the complements of blue, green, and red. This is made necessary by the fact that while the lights

118

Take a picture of these colors ABC

1

Ijp) Unexposed

Yellow

Magenta

Cyan

-

2

Exposed emulsion

emulsion

-

|

_

The picture turns out like this

4

n

5

How a Kodachrome is made

combine to form additive mixtures, the dyes, serving as filters for transmitted light, combine to form subtrac¬ tive mixtures. To see how the system works out, let us trace what happens when you photograph some¬ thing blue, or green, or red. A blue object sends out blue light. (See column A,

119

page 119.) This light exposes only the first emulsion. When the film is developed, the first layer remains clear. The second layer is dyed magenta, and the third layer is dyed cyan.

(See diagram 4, column A.) When

white light is sent through the transparency, the cyan layer removes the red light, and the magenta layer removes the green light. So only the blue light passes through, and we have a blue picture of the blue object. (See diagram 5, column A.) A green object sends out green light. (See column B, page 119.) This light exposes only the second emul¬ sion.

When the him is developed, the second layer

remains clear. The first layer is dyed yellow, and the third layer is dyed cyan. (See diagram 4, column B.) When white light is sent through the transparency, the cyan layer removes the red light, and the yellow layer removes the blue light. So only the green light passes through, and we have a green picture of the green object.

(See diagram 5, column B.)

A red object sends out red light.

(See column C,

page 119.) This light exposes only the third emulsion. When the film is developed, the third layer remains clear.

The first layer is dyed yellow, and the second

layer is dyed magenta.

(See diagram 4, column C.)

When white light is sent through the transparency, the magenta layer removes the green light, and the yellow layer removes the blue light.

So only the red light

passes through, and we have a red picture of the red object.

(See diagram 5, column C.)

120

X Interpreting Color

Color and Meaning

C

olors are found in everything we see. In some

of our experiences we meet the same colors over and over again.

After a while, the colors are

joined in our minds to these experiences. In this way, colors begin to take on special meanings for us. The meanings include such different things as distance, size, weight, temperature, and mood. Color and Depth Have you ever looked at pictures through a stereo¬ scope?

The pictures you see through it look real,

because they show depth. You feel as though you could reach your hand out and touch the things in the pic¬ ture. You get the same sense of depth when you look at a color transparency. When you look at a real scene, there are three clues, among others, that help you see the depth in the scene.

121

The first clue is in the shadows in the scene.

The

second clue comes from seeing two scenes, one in each eye, which are joined together in your mind.

The

third clue is in the colors in the scene. A black and white photograph shows the shadows. But the shadows alone are not enough to give you the feeling of depth. A stereoscope gives you the sense of depth by adding the second clue, of a two-eyed view.

A color trans¬

parency gives you the sense of depth by adding the third clue of color. Near and Far Colors If you look at a blue poster that has words printed on it in red ink, the words seem to step out in front of the poster.

This is because red things always look

nearer than blue things.

Here is the reason why.

When we look at an object, the light from the object enters our eyes and passes through the lens in each eye. The lens bends the light to form an image of the object on the retina of the eye. The nearer an object is to us, the more the light has to be bent to put the image on the retina. To bend the light more, our eye muscles push the edges of the lens in to make it thicker.

So

we can tell that an object is near because we feel our eye muscles working hard when we look at it. In Chapter II we saw that a prism bends blue light more than it bends red light. The lens of the eye, too, bends blue light more easily than red.

To put the

image of a red object onto the retina, the eye muscles

122

have to work extra hard, as if the object were near the eye. That is why red objects seem to be closer than they really are. For a similar reason, a room painted red looks smaller than a room painted blue. Warm and Cool Colors Some colors seem warm to us, while other colors look cool. Yellow, orange, red, and red magenta are warm colors. Greenish yellow, green, cyan, blue, and bluish magenta are cool colors. This may be because the first set of colors reminds us of the hot sun or a flame, while the second set reminds us of cool water or of the sky at twilight, when the air is cool. The employees of a factory once complained that their air-conditioned rest room was very cold.

The

manager checked the temperature of the room and found that it was high enough to be comfortable. How¬ ever, he noticed that the walls of the room were painted blue.

He had the walls painted brown and orange

Which small square is larger? 123

instead. After that, the employees said that the room was warmer, although there was no change in the temperature at all. Brightness and Size In the drawing on page 123 you see a white square on a black background, and a black square on a white background. The white square looks larger than the black square, although they are both the same size. Bright things always tend to look larger. You see another example of the same effect when you turn on an incandescent lamp.

The filament of

the lamp seems to grow wider as it becomes brighter. Color and Weight There is an almost opposite effect in the way in which colors suggest weight. A dark color, which re¬ flects little light, makes something look heavy.

A

bright color, which reflects a lot of light, makes some¬ thing look light in weight. Color and Mood Certain colors fit into a particular mood, and help to create the mood.

Colors that are bright and have

low chroma look light and clean. Colors that are bright and have high chroma look gay. If the colors are dark and have high chroma they suggest richness and qual-

124

ity. But if the colors are dark and have low chroma, they look dull and depressing. A decorator keeps these facts in mind when he chooses the colors for a room. An artist, about to make a painting, has to remember them, too. More Than Meets the Eye We have seen that there is more to color than meets the eye. To find out how colors are made and used, we have had to take short excursions into physics, chemis¬ try, biology, physiology, and psychology. The knowl¬ edge we have gathered from these different areas of learning should help you see the colors around you in a new light. It will give you a deeper understanding of the color in your life.

125

INDEX

Absorption bands, 63 Absorption of light, 22-3, 27, 34, 62, 81-2 Adaptation, 103 Additive mixture of colors, 96-7, 113-4 Afterimage, 105 Alizarine, 76, 84 Anthocyanin, 72 Anthracene, 79-80, 84 Atoms, 57-62, 75-9 excited state of, 60-1 Auxochrome, 83 Azo linkage, 83-4 Benham disk, 101-3 Benzene, 79-80, 84 Black, 27, 36, 91 Bluff, 73 Brightness, 95-6, 104-5, 124 Camouflage, 73 Carbon, 78-9 Carmine, 76 Carotene, 70, 72 Chlorophyll, 70-1 Christensen filter, 47-8 Chroma, 95-6 Chromate, 58 Chrome yellow, 75 Chromium oxide, 75 Chromophores, 83 Cloud, color of, 44-5 Cobalt blue, 75 Color blindness, 108 Color circle, 93-4 Color deepening, 80-2 Colorless, 24 Color mixing, 91-3, 96-9, 113-4 Color photography, 114-20 Color printing, 113-4 Color vision, 106-7 Complementary colors, 92 Darkness, 103 Daylight, color of, 45-6, 53

Depth perception, 121-3 Diffraction, 68-9 Diffraction grating, 49-52, 68 Disguise, 73 Doppler effect, 55-6 Dye, permanence of, 83, 85 Electromagnetic waves, 15, 18, 19 Electrons, 57-60, 64 Eye, 87-90 color of baby’s, 68 Filters, 28-33, 40, 47, 91, 116-7 Flame test, 61-2 Fluorescence, 65 Fovea, 88-90 Frequency, 17, 61 Gray, 29, 91 Haemoglobin, 70 Haltone, 109, 113 Hawk moth, 73 Helmholtz, Hermann, 106-7 Hue, 93-4 Hyacinth, changing the color of, 72 Hydroxyl, 58, 83 Incandescent, 64 Indigo, 75-6, 85-6 Infra-red rays, 17-8, 81 Interference, 69 Iridescence, 68-9 Iris, 67-8 Iron oxide, 74 Kodachrome process, 114-20

106-7,

Lake, 33 Land, Edwin H., 107 Lens, 88-9 Leuco compound, 85-6 Light, speed of, 17 Material medium, 10-11, 40 Mauvine, 78 Maxwell top, 100-2

126

Ring molecules, 79-80, 83 Rods and cones, 89-90, 106-7

Melanin, 71-2 Millimicron, 16 Molecule, 57-8, 62 Monarch butterfly, 73-4 Mother of pearl, 68 Munsell color tree, 95 Murex, 75-6

Scattering of light, 45-6, 67-8, 87, 90 Shells of atoms, 58-9, 78-9 Sienna, 74 Smoke, color of, 46-7 Soap bubbles, 52-3 Spectral colors, 92-3 Spectrophotometer, 63 Spectrum, 14, 19, 51, 65, 81-2, 93 Subtractive mixture of colors, 96-9, 113-4 Sunlight, colors in, 13-5, 39-40, 43-5, 51 Synthesize, 77 Synthetic dyes, 79

Naphthalene, 79-80 Neutron, 57 Nicol prism, 54-5 Nitrobenzene, 79-80 Nucleus, 57-9 Ochre, 74-5 Opal, 69 Opaque, 30

Television, 110-3 Temperature, measurement of, 65 Tomato caterpillar, 73 Translucent, 25 Transmission of light, 22-3, 27, 34-5, 63-4, 81-2, 87 Transparent, 24-5 Tungsten lamp, 64

Peacock, color of, 69 Perkin, William Henry, 78 Persistence of vision, 99 Phonograph record, 51 Phosphor, 110 Photon, 60-5 Pigment, 32-7, 66, 69, 72, 74, 77, 98 mineral, 74-5 organic, 74, 76-8 Pointillism, 99 Polarized light, 54 Primary coiors, 94-5, 113 Prism, 13-5, 18, 39, 43, 54 Proton, 57-8 Prussian blue, 75

Ultra-violet rays, 17-8, 61, 66, 70-3, 81 Valance bond, 78-9 Vermilion, 75 Viceroy butterfly, 73-4 Vitamin A, 70

Quartz, 54-5 Radical, 58 Radio waves, 18 Rainbow, 14-5, 51-2 Reflection of light, 22-3, 26, 31-5, 90-1 Refraction, 42-3, 67-8 Refraction index, 40-1, 44, 48 Retina, 88-9, 103, 106-7

Wave crest, 15-6, 48-50 Wave length, 16 Wave trough, 15-6, 48-50 White, 26, 63 Woad, 76 Wohler, Friedrich, 78 X-rays, 18 Young, Thomas, 106-7

127

■

*

!

.

'

‘

•

$

If you have ever wondered about

COLOR IN YOUR here are the answers to your questions, uj Q Q i—•

Is color in the object you look at, or in the light, or i g Why are clouds white, the sky blue, and the sunset 3 Why are growing things green ? How can two beams of light combine to make dark— •

(CHAPTER,V>

Why is a sidewalk darker when it’s wet?

(chapteriii)

Why are automobile fog-lights yellow?

(chapter iv)

What do "white” and "black” mean?

(chapter iii)

Why does an LP record give a rainbow effect?

(chapter iv)

How can a ray of light be shut off like water from a spigot ? (chapter iv)

How is light made ?

^

(chapter v)

Why are babies’ eyes blue ? And why does their color often change ? (chapter* vi)

What is the difference between camouflage, disguise, and bluff? 1

How can black and white turn into color ?

(chapter VI) (chapter viii)

Why does color make things look closer or more distant, larger or smaller, gayer or duller, warmer or cooler, heavier or lighter ? (chapter x)

U*

b

o £

t JOHN DAY,22 * fiWUUMTEED

t§

A JOHN DAY book