The Nobel Prize in Physics 1921 was awarded to Albert Einstein "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".
Albert Einstein received his Nobel Prize one year later, in 1922. During the selection process in 1921, the Nobel Committee for Physics decided that none of the year's nominations met the criteria as outlined in the will of Alfred Nobel. According to the Nobel Foundation's statutes, the Nobel Prize can in such a case be reserved until the following year, and this statute was then applied. Albert Einstein therefore received his Nobel Prize for 1921 one year later, in 1922.
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Albert Einstein never won a Nobel prize for the theory of relativity—in fact, it was only through long, political jockeying within the Nobel committee that he won the prize at all. Instead, when he was given the 1921 Nobel Prize in Physics (in 1922, after a long bout of internal Nobel hand-wringing), he received it primarily for his explanation of the photoelectric effect. Extraordinarily enough, he came up with both his relativity theory, and the photoelectric effect in the same year: 1905.
At the turn of the century, physicists already knew that, in some circumstances, exposing certain materials to light could create an electric current. An American named Charles Fritts had even created a working solar cell from selenium more than two decades before, in the early 1880s.