Published in the journal Annalen der Physik in 1905, this paper proposed that light is not purely a continuous wave but behaves, in some circumstances, as though its energy is concentrated in discrete packets, later called photons. Einstein reached the idea by comparing the entropy of low-density radiation with that of a gas of independent particles.

He described the proposal as heuristic: a working assumption that might prove useful rather than a settled theory. He then applied it to three phenomena that wave theory struggled to explain, among them the photoelectric effect, in which light striking a metal surface releases electrons.

The paper predicted that the energy of those electrons depends on the frequency of the light rather than its intensity. Robert Millikan confirmed the relationship experimentally over the following decade, and Einstein received the 1921 Nobel Prize in Physics for the work.