Photoelectric effect
Light knocking electrons out of a metal — explained by Einstein with light quanta (1905).
When light hits a metal, it can eject electrons — but only if the light's frequency is high enough, no matter how bright it is. Classical wave theory could not explain this.
In 1905 Einstein proposed that light comes in discrete quanta (photons) of energy E = hf. This explained the effect, helped found quantum theory, and won him the 1921 Nobel Prize.
Source: Wikipedia: Photoelectric effect — Read the full article on Wikipedia.