The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays.
Upon exposing a metallic surface to electromagnetic radiation that is above the threshold frequency or threshold wavelength (which is specific to the type of surface and material), the photons are absorbed and current is produced.
No electrons are emitted for radiation with a frequency below that of the threshold, as the electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface.
By conservation of energy, the energy of the photon is absorbed by the electron and, if sufficient, the electron can escape from the material with a finite kinetic energy.
A single photon can only eject a single electron, as the energy of one photon may only be absorbed by one electron.
The electrons that are emitted are often termed photoelectrons.