2 3 4 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
transferred onto the electron in this collision must therefore be taken from the col-
liding quantum, so that the scattered quantum therefore must have a lower ener-
gy—expressed according to the wave theory, a lower frequency—than the imping-
ing radiation. More careful consideration shows that this frequency diminution of
the scattered radiation is exactly calculable. The percentage change in frequency
comes out very small for visible light; for harder Röntgen radiation, however,
which is, of course, nothing but very short-waved light, it is quite substantial.
Compton has now found that the Röntgen light scattered by suitable substances
does, in fact, manifest the change in frequency demanded by quantum theory (but
not by the wave theory). This can be explained as follows: According to the Ruth-
erford-Bohr theory, every atom possesses a number of electrons that are so loosely
bound to it that they react to the quantum collision of the Röntgen ray roughly as
if they were freely mobile. Hence the above consideration applies to the light scat-
tered by this collision. The positive outcome of Compton’s experiment proves that
radiation acts as if it were composed of discrete projectiles of energy, not just as
regards energy transfer, but also as regards the collision effects.