Photoelectric effect

Photoelectric effect (properly photoemissive effect), the emission of electrons from a surface when struck by electromagnetic radiation, such as light. In 1905 Albert Einstein laid one of the twin foundations of quantum theory by explaining photoemission in terms of the action of individual photons, which are quanta of light. The effect is used in phototubes (electron tubes having a photoemissive cathode), often employed as “electric eye” switches.