For what discovery did Albert Einstein receive the 1921 Nobel Prize in Physics?

Albert Einstein received the 1921 Nobel Prize in Physics for his discovery of the law of the photoelectric effect.

The photoelectric effect occurs when light causes electrons to be emitted from a material. Einstein explained it in 1905 by proposing that light is absorbed in discrete packets of energy, later called photons. The energy of each photon depends on the light's frequency.

Einstein's Nobel citation specifically referred to his contributions to theoretical physics and especially the photoelectric effect; it did not award him the prize for relativity. His explanation helped establish quantum theory and was later supported by experiments.

The prize was awarded in 1922, although it was designated for the 1921 award year. Einstein's work on special and general relativity became more famous publicly, but the photoelectric explanation was the Nobel-recognized achievement.