D O C . 2 3 6 T H E C O M P T O N E X P E R I M E N T 2 3 3
when the waves start to mount? The ships positioned close to the source will keel
over or be smashed, but no harm will come to the ships sufficiently far away from
it; those ships will just rock harmlessly. One would think that molecules struck by
radiation would respond analogously to ships struck by ocean waves. Whether or
not molecules are changed chemically ought to depend not just on the wave length
but also on the intensity of the acting radiation; this is in fact what experience does
not confirm.
Given this failure of the general theory, the light quantum
hypothesis[5]
was re-
sorted to. Notwithstanding all due respect for the wave theory, a working hypothe-
sis gained ground that radiation behaves in energetic respects as if it were com-
posed of energy projectiles whose energy magnitude depends only on the
radiation’s frequency (color) and is proportional to it. Newton’s corpuscular theory
of light is being revived even though it has failed completely with respect to the es-
sentially geometrical properties of light.
Thus, there are now two theories of light, both indispensable and—as has to be
conceded today, despite twenty years of immense efforts by theoretical
physicists—without any logical connection. Quantum theory has made possible
Bohr’s theory of the atom, and has explained so many facts, that there must be a
great amount of truth to it. Given this situation, the question of highest importance
is: to what extent should a projectile-like quality be assigned to light corpuscles, or
quanta?
A projectile transfers not only energy to the struck obstacle but also an impulse
in its direction of motion. Is it likewise for light quanta? On the basis of theoretical
considerations, the response to this question has long been “yes,” and Compton’s
experiment has demonstrated the correctness of this foresight. For a thorough grasp
of this experimental method, one must have a closer look at the mechanism of a
process known as “scattering,” upon which is based, e.g., the blue color of the sky.
If an electromagnetic wave meets a free charged elementary particle (electron)
or one bound to an atom, then the particle is set in oscillatory motion by the wave’s
alternating electric fields. In turn, it thereby emits waves of the same frequency in
every direction (like an antenna in wireless telegraphy) whose energy is taken from
the original wave. The effect of this is that the light irradiated through the medium
containing such particles is scattered (at least in part) in every direction, that is, the
more intensely, the shorter the wave length of the primary light. This is how scat-
tering is interpreted according to the wave theory.
This process is interpreted differently according to quantum theory. According
to it, a light quantum collides with the electron, whereupon it changes its direction
and at the same time imparts a velocity to the electron. The kinetic energy that is