What is the Photoelectric effect?

The photoelectric effect is the stimulated emission of electrons from a (generally metallic) material upon irradiation with light of sufficient energy (light of a short enough wavelength). The incident photons are absorbed by the material, specifically the electrons orbiting the nucleus of the constituent atoms. Depending on the energy of the photon and the type of material being irradiated, the electron may move to a higher energy state, in which, the electron remains constrained to the nucleus still. If, however, the energy of the absorbed photon is large enough, such that the electron has gained enough energy to overcome the attractive electrostatic forces which hold the electron in orbit around the nucleus, the electron is ejected from the nucleus and is ejected into free space, this is known as photoelectric emission. The energy required to overcome the attractive electrostatic forces between the electron and the nucleus is called the workfunction (often denoted by phi). Knowledge of the incident photon's energy (or wavelength/frequency) and the workfucntion of the material, can enable an estimation of the photoelectron's energy and hence its velocity after emission (or vice versa).