Whose theory explained the photoelectric effect?

The photoelectric effect is the release of electrons from a metal when light shines on it, a phenomenon that puzzled physicists because it did not fit the idea of light as a simple wave. In 1905 Albert Einstein proposed that light is made of discrete packets of energy, later called photons, and that each photon can knock an electron loose if it carries enough energy. This explained why the effect depended on the colour, or frequency, of the light rather than its brightness. The idea was a key step in the birth of quantum physics. Einstein received the 1921 Nobel Prize in Physics largely for this work, not for relativity.