The photoelectric effect is a quantum-mechanical phenomenon in which light impinging on the surface of a metal can cause electrons to be ejected. Only light with wavelengths shorter than some threshold value
, characteristic of each metal, can cause emission of photoelectrons, no matter how intense the radiation. Einstein explained this in 1905 by proposing that light is composed of discrete photons, each carrying energy
. Only when the photon energy exceeds the work function
of the metal, a measure of how strongly the outermost electrons are bound, can photoelectrons be emitted. The relevant equation is
, where the last term represents the maximum kinetic energy of the ejected electrons. Once the threshold wavelength is attained, the current of electrons increases linearly with the radiation intensity. This can be monitored by an ammeter in the circuit shown. The light source covers the entire visible range 400–700 nm. In the ultraviolet region, the light ray appears as black.