Table Of ContentINTRODUCTION TO
MODERN PHYSICS
INTERNATIONAL SERIES IN PHYSICS
F. K. RIGHTMYER, CONSULTING EDITOR
Condon and Morse
QuANTUM MECHANICS
Ruark and Urey-
ATOMS, MOLECULES AND QUANTA
Pauling and Goudsmit-
THE STRUCTURE OF LINE SPEC1 RA
INTRODUCTION TO
MODERN PHYSICS
BY
F. K. RICHTMYER
ProfeBsor of Phys·ics at Cornell University
FIRST EDITION
FIFTH. IMPRESSION
McGRAW;..HILL. BOOK COMPANY, lNc.
N,EW TORK A,N'D LON.DON .
1928
COPYRIGHT, 1928, BY THE
McGRAW-HILL BooK COMPANY, INC.
PRINTED IN THE UNITED STA'£ES OP AMERICA
THE MAPLE PRESS COMPANY, YORK, PA,
To !vly Wife
PREFACE
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
Theori.es." These lectures have been adapted, as far as possible,
to meet the needs of two groups of students: (1) those special
students in physics 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 pro
fessions directly or indirectly related to physics, such as engi
neering, 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
vii
PREFACE
Vlll
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."
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 surround
ing 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 morn
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,
there has been no attempt to bring the discussion strictly up to
date. Indeed, with the present rapid growth of physics, i1;
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
prmciples of calculus, for no account of modern physics can
dispense with at least a limited amount of mathematical dis-
PREFACE lX
cussion, 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 questionable from a precise stand
point, have been introduced. These elisions should cause no
confusion.
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 list 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 develop
ments will give the student a better understanding of the subject
than can, in general, be gained by textbook study only. Accord
ingly, he will find here and there throughout this book suggestions
of important articles which should be read in the original. Like
wise, 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 con
cepts. For example, one finds that, taken by and large, the
evolution of physics has been characterized by continuity. That
