What is the emission of electrons from a metal when light shines on it called?

The emission of electrons from a metal when light shines on it is called the photoelectric effect.

Light transfers energy in packets called photons. An electron escapes only when a photon has enough energy to overcome the metal’s work function, the minimum energy needed to remove an electron from its surface. Photon energy increases with frequency, so a threshold frequency must be reached before emission begins.

This effect challenged classical wave theory. Increasing the intensity of low-frequency light does not give each photon enough energy to eject an electron; it mainly increases the number of photons and therefore the number of emitted electrons. Above the threshold, higher frequency produces faster electrons, while greater intensity produces more electrons.

Albert Einstein explained the effect in 1905 using light quanta, helping establish quantum theory. The effect is used in photomultiplier tubes, image sensors, light meters, and photoelectron spectroscopy. It is often confused with the photovoltaic effect, which produces a voltage in a material rather than ejecting electrons into free space.