What is the name of the effect where a metal emits electrons when illuminated with light?

The effect in which a metal emits electrons when illuminated with light is the photoelectric effect.

When light strikes a suitable material, photons can transfer their energy to electrons. If a photon’s energy exceeds the metal’s work function—the minimum energy needed to remove an electron—the electron escapes as a photoelectron. Increasing the light’s frequency increases the maximum kinetic energy of emitted electrons, while increasing intensity mainly increases the number of emitted electrons.

The effect challenged classical wave theories of light because emission can begin almost immediately, with no expected waiting period for energy to build up. Albert Einstein explained the observations in 1905 by treating light as packets of energy, later called photons. His work helped establish quantum theory and earned him the 1921 Nobel Prize in Physics.

The photoelectric effect is often confused with the photovoltaic effect. Photoelectric emission ejects electrons from a material, whereas photovoltaic devices produce a voltage and current within a semiconductor. Thermionic emission uses heat, and the Compton effect involves photon scattering rather than electron emission from an illuminated surface.