Throughout the years of 1909-1911, Einstein studied the "Photoelectric Effect," a paradox regarding light. He resolved it by discovering that light simultaneously behaves as a wave and a particle; this discovery became one of the foundations of Quantum Mechanics, which remains as the best theory to describe small-scale physics. He won the Nobel Prize for this work (and explicitly not for his work in relativity, which was considered at the time to be too radical) in 1922.