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Why did Albert Einstein receive the Nobel Prize in Physics in 1921?

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Portrait of Albert Einstein in a suit and tie, looking slightly to the side with a neutral expression. — Albert Einstein Albert Einstein developed the special and general theory of relativity, which recast gravity as a bending in space-time.

Albert Einstein was awarded the Nobel Prize for Physics in 1921 specifically for his explanation of the photoelectric effect.12

While Einstein is widely celebrated for his theories of relativity, the Nobel committee chose to honor his 1905 work on the photoelectric effect. In this research, Einstein proposed a radical corpuscular theory of light, suggesting that light consists of individual particles—later called photons—each carrying a specific amount of energy, or quantum, proportional to the light's frequency.234

This model successfully resolved inconsistencies in classical physics regarding how light interacts with matter, such as why the energy of ejected electrons depends on the frequency of light rather than its intensity. Although his photon hypothesis was initially controversial, it was later confirmed by precise experimental measurements, leading to his recognition by the Nobel committee.4