What phenomenon ejects electrons from a metal when light shines on it?

The photoelectric effect is the phenomenon in which light ejects electrons from a material, especially a metal.

The effect occurs when incident photons have enough energy to overcome the material’s work function, the minimum energy needed to release an electron. Increasing light frequency raises the maximum kinetic energy of emitted electrons, while increasing intensity mainly increases the number of emitted electrons when the frequency is already above the threshold.

Albert Einstein explained the effect in 1905 by proposing that light transfers energy in discrete packets, later called photons. This work helped establish quantum theory and contributed to Einstein receiving the 1921 Nobel Prize in Physics.

A common mistake is assuming that brighter light always ejects electrons. Below the threshold frequency, even intense light may fail to produce emission in the idealized photoelectric setup.