What is the photoelectric effect?

The photoelectric effect refers to the emmision of electrons from a surface, usually a metal, as a result of shining light (EM radiation). The light is absorbed by the electrons on the surface and thus the electrons gain energy. If the frequency of the light hence their energy is high enough, the electron that absorbes it gaines enough energy and leaves the atom. Different metals have a different threshold frequency, the minimun frequency of the EM radiation required to give enough energy to the electron. Note that it is the frequency/wavelengths of the light that will determine whether electrons will be emitted, and the intensity of the light that will determine how many electrons are emmited