Which physicist won the 1921 Nobel Prize in Physics for explaining the photoelectric effect?

Albert Einstein won the 1921 Nobel Prize in Physics for explaining the photoelectric effect.

In his 1905 explanation, Einstein proposed that light transfers energy in discrete packets, later called photons. An electron is emitted from a metal only when incoming light exceeds a threshold frequency, regardless of how bright the light is.

This work helped establish quantum theory and explained observations that classical wave theory could not. Einstein received the prize in 1921, although the award ceremony took place in 1922.

The prize citation specifically referred to his services to theoretical physics and especially his discovery of the law of the photoelectric effect. His theory of relativity was not the cited achievement, a common misconception about this Nobel Prize.