Concept:
Albert Einstein was awarded the 1921 Nobel Prize in Physics. While he is most famous for relativity, the citation specifically recognized his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.
\textbf Understanding the Significance.
In 1905 (his "Annus Mirabilis"), Einstein proposed that light consists of discrete packets of energy called "quanta" (later photons). This successfully explained why the energy of ejected electrons depended on light frequency, not intensity—a phenomenon classical physics could not explain.