THE BEGINNINGS OF THE QUANTUM THEORY
THE PHOTOELECTRIC EFFECT
Although the term "laser"
can be used for all such devices employi ng
pho tons, it.is common practice to regard a device operating in the microwave
region as a maser and one operating at optical frequencies as a laser.
th is distinction cannot be held
too rigidly, since it is possible to pum p at
microwave frequencies an d to amplify at optical frequencies, or vice versa.
Since there are regi ons of the electromagnetic spectrum wherein it is extremely
difficult either to obtain a mon ochromatic source or to amplify an existing
signal, it would
be attractive to devise lasers which could operate at these
frequencies but which could be pumped at frequencies th at are readily attain-
able (such as "white" light).
T his goal provides impetus for a great deal of
research on the energy levels of ato mic, molecular and man y-body systems."
THE PARTICLE NATURE OF PHOTONS
The measurement of the charge of the electron by Millikan" in 1909
established the fact that charge, as well as energy, is quantized.
That is, any
accumulation of charge mu st consist of an integral mu ltiple of the electronic
charge. Once the value of the electronic charge was known, it was possible to
determine the electron's mass from th e value of
ob tained by T homson-?and
As a result of a great deal of research with cathode ra y tu bes, a number
of interesting demonstrations were devised to verify the particle nature of the
The electron's mass is rn
9.109 x 10-
gram and its charge is
= 4.803 x 10-
statcoulomb = 1.602 x 10-
is easy to accept the particle nature ofthe electron because we can define
its mass, we can accelerate it, and we can make it behave as we think a particle
ough t to behave.
A photon, on the other hand, has no rest mass and cannot be
however, we have seen that it has momentum associated with it
in both classical electromagnetic theory and in the special theory of relativity
(see Equation 1.18).
W e will now discuss two very important experimental
events which can be best explained by assuming that the electron interacts with
a single photon as if the photon were a localized particle rather than a wave
th e quantum or particle nature of light dominates its wave
nature in these experiments. The first is th e well-known
the second is th e
T HE PHOTOELECTRIC EFFECT
Light incident upon a metal surface can, under some conditions, eject
electrons from the surface.
These electrons are called photoelectrons, not that
they differ from other electrons, but
merel y to ide ntify their source.
following facts mu st be explained by a satisfactory theory of the photoelectric
Introduction to Laser Physics.
J ohn Wiley and Sons, Inc. New York,
Lasers and Light, Readingsfrom Scimufi« American.
W . H. Freeman an d Co., San Francisco, 1969.