James Franck won the 1925 Nobel Prize in Physics for discovering the laws governing the impact of electrons upon atoms.
Franck shared the prize with Gustav Hertz. Their experiments fired electrons through mercury vapor and measured how the electrons lost energy in collisions. The results showed that atoms absorb energy in discrete amounts, supporting the emerging quantum theory of atomic energy levels.
The characteristic energy loss corresponded to transitions between allowed states in mercury atoms. This became known as the Franck–Hertz experiment and provided an important experimental confirmation that atomic energy levels are quantized.
The experiment is sometimes confused with the photoelectric effect, which earned Albert Einstein the 1921 Physics prize. Both phenomena involve energy exchange in quantum physics, but Franck and Hertz studied electron collisions with atoms rather than electrons emitted from illuminated surfaces.