Who explained the photoelectric effect by proposing light quanta?

Albert Einstein explained the photoelectric effect by proposing that light comes in discrete packets called light quanta. These packets are now called photons.

In 1905, Einstein argued that each photon carries energy proportional to the light's frequency: E = hν, where h is the Planck constant. When a photon strikes a material, it can transfer its energy to an electron. If the photon has enough energy to overcome the material's work function, the electron is released.

This explained why brighter light does not necessarily produce faster photoelectrons, while higher-frequency light does. It also explained the existence of a threshold frequency: below it, no electrons are emitted regardless of intensity. Max Planck had introduced energy quanta in his work on black-body radiation, but Einstein applied the idea directly to light itself.

The proposal initially challenged classical wave theory. Robert Millikan's experiments in 1914 confirmed Einstein's quantitative prediction, and Einstein received the 1921 Nobel Prize in Physics largely for this work.