Conservation of momentum and energy requires that when a photon collides with a stationary electron, it loses energy.  
It is strong evidence for quantum theory because an explanation requires one to treat light as photons rather than as waves. Conservation of momentum and energy requires that when a photon collides with a stationary electron, it loses energy.  For ten points, what effect named for an American physicist is usually important only for gamma and X-rays?
It is strong evidence for quantum theory because an explanation requires one to treat light as photons rather than as waves.
Compton effect (don't accept inverse Compton)