The electron, its isolation and measurement and the determination of some of its properties

by Robert Andrews Millikan

for energy. Or, if preferred, the same expression for momentum may be deduced easily from the established laws of light-pressure.

The qualitative results of the preceding assumptions are immediately seen to be as follows. The light-quanta, by colliding with the free electron necessarily transfers some of its energy to it, and therefore, if it arrives with the energy

, it must recoil from the impact at some angle

with a smaller energy

, and therefore a lower frequency

, than that with which it impinged. In other words, light waves should be changed from a higher frequency to a lower—from blue toward red—by impact with a free electron.