Introduction To Modern Physics [Third Edition, Fourth Impression]

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IN TERN ATION AL SERIES IN PHYSICS LEE A. DuBRIDGE,

CONSULTING EDITOR

INTRODUCTION TO MODERN PHYSICS

This book is produced in full compliance with the government's regulations for conserving paper and other essential materials.

INTERNATIONAL SERIES IN PHYSICS LEE A. DuBRIDGE,

Bacher and Goudsrnit-ATOMIC

Consulting Editor

ENERGY STATES

Bitter-INTRODUCTION TO FERROMAGNETISM

Clark-APPLIED X-RAYS Condon and M orse-QuANTUM MECHANICS Curtis-ELECTRICAL MEASUREMENTS Davey-CRYSTAL STRUCTURE AND !Ts APPLICATIONS Edwards-ANALYTIC AND VECTOR MECHANICS Eldridge-THE PHYSICAL BASIS OF THINGS Hardy and Perrin-THE PRINCIPLES OF OPTICS Harnwell-PRINCIPLES OF ELECTRICITY AND ELECTROMAGNETISM

Harnwell and Livingood-EXPERIMENTAL ATOMIC PHYSICS Houston-PRINCIPLES OF MATHEMATICAL PHYSICS Hughes and DuBridge-PHOTOELECTRIC PirnNOMENA Hund-HIGH-FREQUENCY MEASUREMENTS

Hund-PHENOMENA IN HIGH-FREQUENCY SYSTEMS Kernble-THE FUNDAMENTAL PRINCIPLES OF QUANTUM MECHANICS

Kennard-KINETIC THEORY OF GASES Koller-THE PHYSICS OF ELECTRON TUBES Morse-VIBRATION AND SouND Muskat-THE FLOW OF HOMOGENEOUS FLUIDS

THROUGH

POROUS MEDIA

Pauling and Goudsrnit-THE STRUCTURE OF LINE SPECTRA Richtrnyer and Kennard-INTRODUCTION TO MODERN PHYSICS

Ruark and Urey-ATOMS, MOLECULES AND QUANTA Seitz-THE MODERN THEORY OF SOLIDS Slater-INTRODUCTION TO CHEMICAL PHYSICS Slater-MICROWAVE TRANSMISSION Slater and Frank-INTRODUCTION TO THEORETICAL PHYSICS

Srnythe-STATIC AND DYNAMIC ELECTRICITY Stratton- ELECTROMAGNETIC THEORY White-INTRODUCTION TO ATOMIC SPECTRA

Williams-MAGNETIC

PHENOMENA

Dr. F. K. Richtmyer was consulting editor of the series from its inception in 1929 until his death in 1939.

INTRODUCTION TO

MODERN PHYSICS BY

F. K. RIGHTMYER Late Professor of Physics at Cornell University

AND

E~H.KENNARD Professor of Physics at Cornell University

THIRD EDITION FOURTH IMPRESSION

McGRAW-HILL BOOK COMPANY, INc NEW YORK AND LONDON 1942

INTRODUCTION TO MODERN PHYSICS COPYRIGHT,

1928, 1934, 1942,

McGRAW-HILL

BY THE

BooK COMPANY, lNc.

PRINTED IN THE UNITED STATES OF AMERICA

All rights reserved. This book, or parts thereof, may not be reproduced in any form without permission of the publishers.

THE MAPLE PRESS COMPANY, YORK, PA.

PREFACE TO THE THIRD EDITION After the sudden death of Professor Richtmyer, it was decided that the writer should undertake the preparation of a new edition of his "Introduction to Modern Physics." Closer inspection showed that something like a major operation would be necessary, so greatly has the scene changed in physical research during the last ten years. In atomic theory, wave mechanics has become thoroughly established; and experimental advance has been rapid in the physics of the nucleus and of fundamental particles, as observed in the phenomena of cosmic rays. If the book was to remain true to its title, extensive additions seemed necessary, and to make room for them the existing material had to be shortened; at the same time, much of the discussion needed recasting in order to give due recognition to the triumph of wave mechanics. In making these changes, the writer has striven to retain as much as possible of the characteristic features of the book. Nevertheless, its friends will no doubt be shocked to discover that only a minor part of it stands substantially as in the second edition. The double process of shortening and of modernizjng the viewpoint was found to necessitate extensive rearrangement and much rewriting of the material. The writer can only hope that he has not fallen too far short of his goal in attempting to produce an up-to-date equivalent of the first edition. Briefly, the course of the revision has been as follows. The historical introduction, abbreviated, is followed by a single chapter, rewritten, on those topics in electromagnetism which are needed for the subsequent discussion and are not always adequately treated in general textbooks. Between the chapters on the photoelectric effect and on the origin of the quantum theory is inserted a short chapter on relativity. Then comes a single chapter, replacing Chaps. IX and X, containing the essential ideas concerning the nuclear atom, spectral series, and atomic quantum states. The Bohr theory of hydrogen is retained, because of its pictorial value, but with a clear statement as to its true status. A descriptive chapter on wave mechanics is then followed by a single chapter, replacing Chaps. XI and XII, on the theory of the periodic table and on optical spectroscopy. The chapter on specific heats, which might perhaps have been· omitted, is inserted next. The chapter on X-rays has been thoroughly revised v

PREFACE TO THE THIRD EDITION

VI

in collaboration with Prof. L. G. Parratt. The book closes with the chapter on the nucleus, considerably extended, and a new chapter on cosmic-ray phenomena. The writer takes pleasure in acknowledging his indebtedness to many of his colleagues in Cornell University, in particular to Prof. L. G. Parratt, to Prof. B. Rossi for assistance in the field of cosmic rays, and to Dr. C. W. Gartlein for photographic copies of illustrations. Grateful acknowledgment is also made for permission to use figures from other sources: to G. Herzberg and to Prentice-Hall, Inc., for Figs. 58, 59, 129, 132, 133; to A. H. Compton and S. K. Allison and to D. Van Nostrand Company, Inc., for Figs. 169, 170; to H. E. White, for Figs. 98, 118, 122; to Prof. C. D. Anderson for photographs for Figs. 226, 227; and to Prof. L. G. Parratt for a cut for Fig. 180. Furthermore, in accord with the statement in the pref ace to the second edition, grateful acknowledgment is made to Dr. K. T. Bainbridge for Fig. 187; to Dr. C. J. Davisson and L. H. Germer (and to the editors of the Bell System Technical Journal and of the Proceedings of the National Academy of Sciences) for Figs. 79 to 84; to Prof. G. P. Thomson (and Cornell University) for Fig. 86(b); to Dr. E. Rupp (and the editors of Annalen der Physik) for Fig. 86(c); and to Dr. I. Fankuchen for making the plate from which Fig. 86(a) was reproduced. Finally, the writer is greatly indebted to Dr. ,v. M. Cady and to other users of the book for invaluable criticisms and suggestions.

E. H. N. Y., January, 1942.

ITHACA,

KENNARD.

PREFACE TO THE FIRST EDITION For several years, the author has given at Cornell University, and, occasionally, in summer sessions, elsewhere, a course of lectures under the title "Introduction to Modern Physical Theories." These lectures have been adapted, as far as possible, to meet the needs of two groups of students: (1) those special students in physjcs who, before entering the specialized graduate courses, desire a survey of the origin and development of modern physics in order the better to understand the interrelations of the ·more advanced courses; and (2) those students who, pursuing either academic or professional curricula and having had the usual elementary undergraduate courses in physics, wish a further bird's-eye view of the whole subject. This book is based upon these lectures and has been prepared, although rather reluctantly, as a result of the importunities of former students and other friends. The purpose of .this book is, frankly, pedagogical. The author has attempted to present such a discussion of the origin, development, and present status of some of the more important concepts of physics, classical as well as modern, as will give to the student a correct perspective of the growth and present trend of physics as a whole. Such a perspective is a necessary basis-so the author, at least, believes-for a more intensive study of any of the various subdivisions of the subject. While for the student whose interests are cultural, or who is to enter any of the professions directly or indirectly related to physics, such as engineering, chemistry, astronomy, or mathematics, an account of modern physics which gives the origin of current theories is likely to be quite as interesting and valuable as is a categorical statement of the theories themselves. Indeed, in all branches of human knowledge the "why" is an absolutely indispensable accompaniment to the "what." "Why?" is the proverbial question of childhood. "Why?" inquires the thoughtful ( !) student in classroom or lecture hall. "Why?" demands the venerable scientist when listening to an exposition of views held by a colleague. Accordingly, if this book seems to lay somewhat greater emphasis on matters which are frequently regarded as historical, or, if here and there a classical experiment is described in greater detail than is customary, it is with a desire to recognize the importance of "why." vii

Vlll

PREF ACE TO THE FIRST EDITION

If one were to attempt to answer all of the "why's" raised by an intelligent auditor in answer to a categorical statement, such as, "The atom of oxygen is composed of eight electrons surrounding a nucleus containing four alpha particles," one would have to expound a large part of physical science from Copernicus to Rutherford and Bohr. To attempt a statement of even the more important concepts, hypotheses, and laws of modern physics and of their origin and development would be an encyclopedic task which, at least in so far as concerns the aim of this book, would fall of its own weight. Accordingly, it has been necessary to select those parts of the subject which best serve our purpose. This selection, as well as the method of presentation, has been based upon the experience gained in giving the above-mentioned lectures to numerous groups of students. Many very important developments, particularly the more recent ones, either have been omitted entirely or have been given only a passing comment. And even in those parts of the subject which have been discussed, ~here has been no attempt to bring the discussion strictly up to date. Indeed, with the present rapid growth of physics, it would be quite impossible for any book, even a special treatise, to be strictly up to date. Happily, for our purpose, up-to-date-ness is not an imperative requisite, since it is assumed that the student who wishes the latest knowledge will consult the current periodicals. In this connection, it should be emphasized that this book is an introduction to modern physical theories and is intended neither as a compendium of information nor as a critical account of any of the subjects discussed. In preparing the manuscript, the author has consulted freely the many very excellent texts which deal with the various special topics. Save for here and there a very minor item, or an occasional novelty in presentation, the book makes no claim to originality, except, perhaps, as regards the viewpoint from which some parts have been written. It is assumed that the student is familiar with the elementary principles of calculus, for no account of modern physics can dispense with at least a limited amount of mathematical discussion, if for no other reason than to emphasize the fact that, in the progress of physics, theory and experiment have gone hand in hand. Partly, however, for the sake of brevity and partly in the attempt always to keep the underlying physical principles in the foreground, numerous "short cuts" and simplifications, some of them perhaps rather questio.nable from a precise standpoint, have been introduced. These elisions should cause no confusion.

PREFACE TO THE FIRST EDITION

lX

The student who, in his educational career, has reached the point where he can, with profit, pursue a course based on such a book as this, has passed beyond the stage where he assimilates only the material presented in lecture or class and has come to regard a "course" as a channel to guide his own independent studies, branching out from the "course" in such directions as his fancy or interests may lead him. It is hoped that students reading this book will do likewise. Deliberately, the·author has not given a collected bibliography at the end of each chapter, or a Ii.st of problems and suggested topics for study. Rather, references, in most cases to original sources, have been given at appropriate points in the text, and it is hoped that, starting from these references, the student will prepare his own bibliography of such parts of the subject as appeal to him. The advantage to the student of such a procedure is obvious. Quite apart from the value of the experience gained in making contact with, and in studying, the literature of any subject, the reading of first-hand accounts of at least some of the more important developments will give the student a better understanding of the subject than can, in general, be gaiP.ed by textbook study only. Accordingly, he will find here and there throughout this book suggestions of important. articles which should be read in the original. Likewise, in many places the discussion has, of necessity, been brief, and the student is referred to special treatises for further details. Various supplementary questions and problems will also arise at numerous points as the student reads the text. There is no more fascinating story than an account of the development of physical science as a whole. (Any scientist would probably make the same statement about his own science!) Such a study leads to certain broad generalizations which are of outstanding importance in evaluating current theories and concepts. For example, one finds that, taken by and large, the evolution of physics has been characterized by continuity. That is to say: With few exceptions, the ideas, concepts, and laws of physics have evolved gradually; only here and there do we find outstanding discontinuities. The discovery of photoelectricity, of X-rays, and of radioactivity represent such discontinuities and are correctly designated "discoveries." But we must use "discover" in a quite different sense when we say that J. J. Thomson ''discovered'' the electron. The history of the electron goes back at least to Faraday. Thomson's experiments are all the more brilliant because he succeeded in demonstrating, by direct experiment, the existence of something, evidence concerning which had been previously indirect. Then, there are the respective roles played by qualitative

x

PREFACE TO THE FIRST EDITION

and by quantitative work. Numerous important discoveries have been made "by investigating the next decimal place." Witness the discovery of argon. And ever since Kepler proved that the orbits of the planets are ellipses, relations expressible in quantitative form have carried greater weight than those which could be stated only qualitatively. For example, Rumford's experiments on the production of heat by mechanical means were suggestive. But Joule's measurement of the mechanical equivalent of heat was convincing. If, directly, or indirectly by inference, the author has succeeded here and there in the text in pointing out such generalizations as these, one more object which he has had in mind will have been accomplished. · The author wishes to take this occasion to acknowledge his obligations to those who have aided in the preparation of this book: to his wife, for assistance in preparing the manuscript and in proof reading; and to his many students, whose generous approbation of the lecture courses upon wh1ch the book is based has, in a large part, inspired its preparation. He. is particularly indebted to Dr. J. A. Becker, of the Bell Telephone Laboratories, Inc., for his invaluable aid in reading the manuscript, pointing out numerous errors, and suggesting important improvements. F. K. RICHTMYER. ITHACA,

N. Y.,

July, 1928.

CONTENTS (Page numbers are given after the section headings.) PAGE

v

PREFACE TO THE THIRD EDITION. . PREFACE TO THE FIRST EDITION . .

Vll

1

INTRODUCTION. . . . . . . . CHAPTER I HISTORICAL SKETCH . . . . . . . . . . . . . . . . . . . . . . . . . First Period: Earliest Times to A. D. 1550: 1. The Greeks (5) 2. Thales of Miletus (5) 3. Pythagoras (5) 4. Anaxagoras and Empedocles (5) 5. Democritus (6) 6. Aristotle (6) 7. Aristarchus (9) 8. Archimedes (9) 9. From the Greeks to Copernicus (9) 10. The Copernican System (11). Second Period (A. D. 1550-1800): Rise of the Experimental Method. 11. Galileo Galilei (12) 12. Tycho Brahe and Kepler (17) 13. The Experimental Method Spreads (19) 14. Sir Isaac Newton (21) 15. Newton's Contemporaries (28) 16. Mechanics during the Eighteenth Century (28) 17. Heat during the Eighteenth Century (29) 18. Light during the Eighteenth Century (29) 19. Electricity during the Eighteenth Century (30) 20. Close of the Second Period (32). Third Period (A. D. 1800-1890): The Rise of Classical Physics. 21. The Nineteenth Century in Physics (32) 22. Heat and Energy (33) 23. Light (34) 24. Electricity and Magnetism (36) 25. Michael Faraday (38) 26. Joseph Henry (45) 27. James Clerk Maxwell (46) 28. The Completion of Electromagnetic Theory (50). CHAPTER II ELECTROMAGNETIC WAVES AND MovING CHARGES . . . . . . . . . 29. Displacement Currents (51) 30. Maxwell's Equations (54) 31. Energy and Momentum in the Electromagnetic Field (59) 32. Electromagnetic Waves (61) 33. Velocity of Electromagnetic Waves (63) 34. Energy of Electromagnetic Waves (65) 35. Momentum of Electromagnetic Waves. Radiation Pressure (68) 36. Field of a Uniformly Moving Point Charge (70) 37. Radiation Field of an Accelerated Point Charge (73) 38. Energy Radiated by an Accelerated Point Charge (76) 39. Electromagnetic Mass (80). CHAPTER III THE PHOTOELECTRIC AND THERMIONIC EFFECTS . . . . . . . . . . . 40. Discovery of the Photoelectric Effect (83) 41. A Problem (85) 42. Electricity in Matter (86) 43. The Zeeman Effect (87) 44. The Xl

5

51

83

CONTENTS

XU

PAGE

Discovery of the Electron (92) 45. Electronic Magnitudes (95) 46. Photoelectrons (97) 47. Relation between Photoelectric Current and Intensity of Illumination of the Cathode (100) 48. Energy Distribution of Photoelectrons (100) 49. Relation between the Velocities of Photoelectrons and the Frequency of the Light (103) 50. Other Properties of Photoelectric Emission (104) 51. Source of the Photoelectric Energy (105) 52. The Photoelectric Effect and the Corpuscular Theory of Light (108) 53. Thermionic Emission (109) 54. Relation between Thermionic and Photoelectric Constants (111) 55. Velocities of Emission of Thermions (112) 56. Theories of Electrons in Metals (113) 57. Origin of Photoelectrons and Thermions (117). CHAPTER IV THE THEORY OF RELATIVITY.

122

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58. Newtonian Relativity (122) 59. Relativity and the Propagation of Light (124) 60. The Michelson-Morley Experiment (126) 61. The New Relativity of Einstein (130) 62. Simultaneity and Time Order (132) 63. The Lorentz Transformation (134) 64. Contractions in Space and Time (136) 65. The Transformation of Velocities (138) 66. General Significance of the Theory of Relativity (139) 67. Relativistic Mechanics. The Variation of Mass (140) 68. Force and Kinetic Energy (143) 69. A Relation between Mass and Energy (145) 70. Relativity and Electromagnetism (147) 71. General Theory of Relativity (150) 72. Einstein's Law of Gravitation (152). CHAPTER V THE ORIGIN OF THE QUANTUM THEORY .

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154

73. Thermal Radiation (154) 74. Some Fundamental Concepts and Definitions (155) 75. The Black Body and the Isothermal Enclosure (160) 76. Relation between Emissive Power and Absorptivity (162) ·77_ Pressure due to Isotropic Radiation (164) 78. Deduction of the Stefan-Boltzmann Law (166) 79. Experimental Verification of the Stefan-Boltzmann Law (169) 80. Reflection from a Moving Mirror (170) 81. Effect of an Adiabatic Expansion upon Black-body Radiation (173) 82. The Wien Displacement Law (175) 83. The Formula for Black-body Radiation (178) 84. Degrees of Freedom (179) 85. Relation between Energy per Degree of Freedom and the Temperature (180) 86. Verification of Equipartition of Energy in the Brownian Motion (183) 87. Degrees of Freedom in an Enclosure (184) 88. The Rayleigh-Jeans Formula (189) 89. Other Formulas for Black-body Radiation (190) 90. Distribution of Harmonic Oscillators in Thermal Equilibrium (191) 91. Average Energy of Oscillators in Equilibrium (194) 92. Planck's Quantum Hypothesis (197) 93. Planck's Radiation Law (200). CHAPTER VI THE NUCLEAR ATOM AND THE ORIGIN OF SPECTRAL LINES.

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94. Spectroscopic Units (207) 95. Early Search for Regularities in Spectra (209) 96. Spectral Series and their Interrelations (212) 97. Ti1,nther Relationships between Series. Spectral Terms (215) 98. Early

206

CONTENTS

Xlll PAGE

Views on Atomic Structure (217) 99. The Thomson Atom (219) 100. The Scattering of Alpha Particles by Atoms (220) 101. Rutherford's Nuclear Atom (222) 102. The Bohr Theory of Atomic Hydrogen (226) 103. Quantum States of One Electron in an Atom (230) 104. Spectrum of a One-electron Atom (233) 105. The Spectrum of Atomic Hydrogen. Energy Levels and Spectral Series (237) 106. Ionized Helium (239) 107. Energy Levels and Series Relationships for Sodium (241) 108. Excitation and Ionization of Atoms by Electrons (243) 109. Absorption and Re-emission of Radiation (24 7) 110. The Boltzmann Distribution Law (255) 111. The Extension of Bohr's Theory (256). CHAPTER VII \VAVE Jv.lECHANICS . . . . . . . • . . . . . . . . . . . . . . . . . .

259

112. Matter \Vaves (259) 113. Mechanics as Geometrical Optics of the \Vaves (262) 114. Refraction of Matter \Vaves (263) 115. Fermat's Princ~ple and the Principle of Least Action (266) 116. The de Broglie \V ave Length (269) 117. Experiments on Electron \V aves (273) 118. Diffraction of Molecule \V aves (283) 119. Schrodinger's \V ave Equation (284) 120. Physical Significance of '¥ (289) 121. The Indeterminacy Principle (292) 122. Stationary States (294) 123. The Harmonic Oscillator (296) 124. The One-electron Atom (297) 125. Emission and Absorption of Radiation (304) 126. Relativistic Effects and Electron Spin (309). CHAPTER VIII ATOMIC STRUCTURE AND OPTICAL SPECTRA.

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\Vave Mechanics of Many-electron Systems: 127. \Vave-mechanical Fundamentals (311) 128. The Zero-order Approximation: Spin Energy Omitted (314) 129. The :M:ore Accurate Theory (319). The Periodic Table of,the Elements: 130. Atomic Numbers (320) 131. Some Features of the Periodic Table (321) 132. The Static Atom (324) 133. \V ave Mechanics and the Periodic Table: the First Two Periods (325) 134. Valence Bonds (329) 135. The Third Period of the Periodic Table (332) 136. The Fourth and Fifth Periods (333) 137. The Periodic Table Concluded (335). Optical Spectra: 138. Atomic and Molecular Spectra (337) 139. Angular Momentum in \Vave Mechanics (339) 140. Total Angular Momentum and its Selection Rules (340) 141. Orbital and Spin Angular Momenta (342) 142. Alkali-type Spectra (345) 143. The Term Energies of the Alkali Metals (350) 144. The Addition of Orbital and · Spin Momenta (353) 145. Number of Levels in an LS Multiplet (355) 146. Spin-orbit Interaction and Multiplet Spacing (358) 147. Fine Structure in Alkali-type Spectra (362) 148. Multiplet Levels for Oneelectron Atoms (366) 149. Fine Structure of Spectral Lines from Oneelectron Atoms (369) 150. Two-electron Spectra (372) 151. The Spectrum of Mercury (377) 152. Equivalent Electrons (379) 153. ''jj" Coupling (380) 154. Spectra due to Three or More Eleotrons (383) 155. The Effect of a Magnetic Field on an Atom (384) 156. Classical Theory of the Magnetic Energy (387) 157. Zeeman Effect in a Huge Field (391) 158. Zeeman Effect in a \Veak Field (396) l 59. Zeeman

311

CONTENTS

XIV

PAGE

Patterns of LS Multiplets in a Weak Field (400) 160. The PaschenBack Effect (403) 161. The Stark Effect (406) 162. The SternGerlach Experiment (407) 163. Isotope Structure and Hyperfine Structure (409) 164. The Breadth of Spectral Lines (414) 165. Molecular Spectra (417) 166. Rotation Spectra (419) 167. Vibrationrotation Spectra (421) 168. General Theory of Molecular Quantum States (429) 169. Electronic Bands (433) 170. The Raman Effect (437). CHAPTER IX THE QUANTUM THEORY OF SPECIFIC HEATS.

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442

171. Variation of Specific Heats of Solids with Temperature (442) 172. Classical Theory of the Specific Heats of Solids (444) 173. Einstein's Theory of the Atomic Heats of Solids (445) 174. Characteristic Frequencies (447) 175. Debye's Theory of Atomic Heats (450) 176. Experimental Test of Debye's Equation (453) 177. Molecular Heats of Mixed Solids (456) 178. The Molecular Heat of Gases: Classical Theory (457) 179. Quantum Theory of the Specific Heat of Gases (460) 180. Comparison of the Theory with Observed Specific Heats (465). CHAPTER X X-RAYS.

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469

Early, Mostly Qualitative Developments in X-rays (1895-1912): 181. Roentgen's Discovery (469) 182. Production and Measurement of X-rays (471) 183. Classical Pulse Theory of X-rays (473) 184. The Scattering of X-rays (475) 185. Absorption and Fluorescence (482). X-ray Spectra: 186. The Crystal Diffraction Grating (486) 187. The X-ray Spectrometer (491) 188. Bragg's Discovery of Monochromatic Characteristic Radiations (495) 189. Moseley's Law (496) 190. The Origin of X-ray Lines (498) 191. X-ray Energy Levels (504) 192. The Quantum Theory of X-ray Terms and Lines (507) 193. The Continuous X-ray Spectrum (512) 194. Intensity of the Continuous Spectrum (517). Interactions of X-rays with Atoms: 195. The Absorption of X-rays (519) 196. The Photoelectric Effect for X-rays (523) 197. The Scattering of X-rays (528) 198. The Compton Scattering of X-rays (531) 199. The Refraction of X-rays (539) 200. Measurement of X-ray Wave Lengths by a Ruled Grating (544). Some Recent Developments in X-ray Spectroscopy: 201. Multiple Ionization of Inner Electron Shells (545) 202. X-ray Spectra and the Outer Part of the Atom (550) 203. X-ray Spectroscopy of Solids (553). CHAPTER XI THE NucLEus.

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The Masses of Atoms: 204. Positive Rays (556) 205. The Spectrograph (559) 206. Isotopes (563). Radioactivity: 207. Becquerel's Discovery of Radioactivity (566) The Radioactive Radiations (567) 209. The Alpha-rays (568) Radioactive Transformations (574) 211. Gamma-ray Spectra Nuclear . Energy Levels (577) 212. Beta-ray Spectra (582) Observations on Individual Charged Particles (583).

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Mass 208. 210. and 213.

556

xv

CONTENTS

PAGE

The Structure of Nuclei: 214. The Building up of Nuclei (586) 215. The General Theory of Nuclear Binding (591) 216. Nuclear Forces and the Ratio of Mass Number to Atomic Number (593) 217. The Explanation of Natural Radioactivity (597). Artificial Transmutation of Nuclei: 218. Artificial Transmutation by Alpha Particles (599) 219. Artificial Transmutation by High-velocity Protons (602) 220. Discovery of the Neutron (606) 221. Positrons (610) ~22. The Discovery of Induced Radioactivity (613) 223. Experiments with Neutrons (615) 224. The Production of High-speed Ions (621) 225. Some Typical Nuclear Reactions (625) 226. Fission of the Nucleus (638). CHAPTER XII COSMIC RAYS

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643

227. Early Work on Cosmic Rays (643) 228. The Measurement of Cosmic-ray Ionization (646) 229. The Altitude-depth Curve (648) 230. Discovery of the Latitude Effect (650) 231. Motion of a Charged Particle in the Equatorial Plane of a Magnetic Dipole (651) 232. Motion of Charged Particles in the Magnetic Field of the Earth (656) 233. The Variation of Cosmic-ray Intensity with Latitude (661) 234. Observations on Single Cosmic-ray Particles (665) 235. Energy, Mass, and Specific Ionization of Charged Particles (669) 236. Showers and Bursts (674) 237. Theory of the Shower Phenomenon (679) 238. Mesotrons (689) 239. Origin and Fate of Mesotrons (693) 240. Conclusion (696). APPENDIXES I.

. 699

ISOTOPIC CONSTITUTION OF THE ELEMENTS

II. THE PERIODIC TABLE. III. FIRST IONIZATION

704

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POTENTIALS

(WHERE

KNOWN), LOWEST SPECTRAL

TERMS, AND ELECTRON CONFIGURATION OF THE ELEMENTS

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705

SOME USEFUL CONSTANTS AND RELATIONS.

. 708

INDEX . . . . • . . • . . . . . . . . .

709

INTRODUCTION TO MODERN PHYSICS INTRODUCTION The term "modern physics," taken literally, means, of course, the sum total of knowledge included under the head of present-day physics. In this sense, the physics of 1890 is still modern; very few statements made in a good physics text of 1890 would need to be deleted today as untrue. The principal changes required would be in a few generalizations, perhaps, to which exceptions have since been discovered, and in certain speculative theories, such as that concerning the ether, which any good physicist of 1890 would have recognized to be open to possible doubt. On the other hand, since 1890, there have been enormous advances in physics, and some of these advances have brought into question, or have directly contradicted, certain theories that had seemed to be strongly supported by the experimental evidence. For example, few, if any, physicists in 1890 questioned the wave theory of light. Its triumphs over the old corpuscular theory seemed to be final and complete, particularly after the brilliant experiments of Hertz, in 1887, which demonstrated, beyond doubt, the fundamental soundness of Maxwell's electromagnetic theory of light. And yet, by an irony of fate which makes the story of modern physics full of the most interesting and dramatic situations, these very experiments of Hertz brought to light a new phenomenon-the photoelectric effectwhich played an important part in establishing the quantum theory. The latter theory, in many of its aspects, is diametrically opposed to the wave theory of light; indeed, the reconciliation of these two theories, each based on incontrovertible evidence, was one of the great problems of the first quarter of the twentieth century. It will be the purpose of the following pages to give an outline of the origin, development, and present status of these parts of physics which have developed during recent decades. But a history of the United States cannot begin abruptly with July 4, 1776. In like manner, if we would understand the full meaning of the growth of physics since 1900, we must have clearly in mind at least the main events in the development of-the subject up to that time. 1

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INTRODUCTION TO MODERN PHYSICS

Accordingly, we shall begin our study by a brief account of the history of physics up to a half-century ago. In presenting this brief historical survey, a further purpose has been kept in mind toward which it is hoped that the reader will be, ultimately at least, sympathetic. Modern scientists, with few exceptions, have grossly neglected to cultivate the history of their respective sciences. How many physicists can answer the questions: When was the law of the conservation of energy first enunciated? vVho was Count Rumford? Did the concept of universal gravitation spring full grown from the head of that genius, Newton? Indeed, when did Newton live? Just as any good American should know the essential outline of the history of his country, so any good physicist should know the principal facts in the history of physics. For, in that history, in the lives of those men whose labors have given us our subject, and in the part that physics has played in molding human thought and in contributing to modern civilization, the student will find a story which is as full of human interest and inspiration as any subject of the curriculum. What can be more inspiring than the life of Michael Faraday and his whole-souled devotion to his work? The physicist owes it to his science to possess such a knowledge of the history of physics as gives him a correct perspective of the development and present-day importance of the subject and, in turn, enables him to acquaint his contemporaries in other fields with these essential facts. It is hoped, therefore, that the student who proposes to follow physics as a profession, as well as the student whose interest is largely cultural, will extend the following all too brief historical sketch by independent study, particularly of biography. In order to make it easier to keep the essential facts in mind, we may somewhat arbi.trarily divide the history of physics into four periods. The FrnsT PERIOD extends from the earliest times up to about 1550 A.D., this date marking approximately the beginning of the experimental method. During this long era, there was some advance in the accumulation of the facts of physics as a result of the observation of natural phenomena, but the development of physical theories was rendered impossible, partly by the speculative, metaphysical nature of the reasoning employed, but more particularly by the almost complete absence of experiment to test the correctness of such theories as were proposed. The main characteristic of this period, therefore, is the absence of systematic experiment. The SECOND PERIOD extends from 1550 to 1800 A.D. Although numerous basic advan