Photoemission is the ejection of electrons from the surface of a material (metal) caused by an increase of energy, such as a photon of light of sufficiently high frequency striking the surface. The inbound photon must have a frequency high enough such that it's energy, given by


where h is Planck's constant and f is the frequency, is high enough so that an electron in the valence band can overcome the metal's work function and be ejected.