For which discovery did Albert Einstein win the Nobel Prize?

Albert Einstein won the Nobel Prize in Physics for explaining the photoelectric effect. The 1921 prize recognized his services to theoretical physics, especially his discovery of the law governing this phenomenon.

The photoelectric effect occurs when light shining on a material causes electrons to be emitted. Einstein proposed in 1905 that light behaves as packets of energy, later called photons. Each photon’s energy depends on the light’s frequency, not merely its brightness. Light below a material’s threshold frequency cannot eject electrons, no matter how intense it is.

This idea challenged the purely wave-based description of light and helped establish quantum theory. The effect was experimentally studied earlier by Heinrich Hertz and others, but Einstein supplied the decisive theoretical explanation. Robert Millikan later measured the effect with great precision, even though he initially regarded Einstein’s light-quantum hypothesis skeptically.

Many readers expect Einstein’s Nobel to have been awarded for special or general relativity, or for E=mc². Those achievements made him famous, but the Nobel citation specifically honored the photoelectric effect. Einstein formally received the 1921 prize during the 1922 Nobel ceremony.