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

E=hf

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.