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)