Actually, that's a common misconception. Einstein won the Nobel Prize in Physics in **1921** for the **discovery of the law of the photoelectric effect**, not for the theory of relativity.

His work explaining the photoelectric effect was foundational to quantum theory. While his theories of relativity (special and general) are arguably more famous, the Nobel Committee at the time was more comfortable awarding the prize for the photoelectric effect, which had more direct experimental confirmation at that point.
