What phenomenon did Einstein explain with his 1905 paper on light?

Einstein explained the photoelectric effect in his 1905 paper on light.

The effect occurs when light strikes a material and ejects electrons from its surface. Experiments showed that light below a certain frequency could not release electrons, even if its intensity was increased. Einstein proposed that light arrives in discrete packets of energy, later called photons, with each packet carrying energy proportional to its frequency.

This explained why frequency determines whether electrons escape, while intensity mainly affects how many electrons are emitted. The electron’s maximum energy follows the relation E = hf − W, where h is Planck’s constant and W is the material’s work function. The idea was controversial because classical physics treated light primarily as a wave.

A common mix-up is confusing this with the Compton effect, which concerns X-ray scattering from electrons. Einstein’s photoelectric-effect work was specifically cited when he received the 1921 Nobel Prize in Physics; later experiments by Robert Millikan and Arthur Compton helped confirm the quantum picture.