Which phenomenon did Albert Einstein explain in his Nobel Prize-winning 1905 paper?

Albert Einstein explained the photoelectric effect in his Nobel Prize-winning 1905 paper.

The photoelectric effect occurs when light causes electrons to be emitted from a material. Einstein proposed that light transfers energy in discrete packets, later called photons. The energy of each packet depends on light frequency, while increasing intensity mainly increases the number of emitted electrons.

This explanation helped establish the quantum nature of light. It also accounted for observations that classical wave theory struggled to explain, including the existence of a threshold frequency and the immediate release of electrons above that threshold.

Einstein received the 1921 Nobel Prize in Physics, awarded in 1922, “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.” Special relativity and Brownian motion were also major achievements of his 1905 work, but neither was the Nobel citation’s focus.