PHOTOELECTRIC effect showing photons striking a metal surface and electrons being ejected. Purple spheres represent photons (e.g. of ultraviolet light) striking a metal surface. These photons "knock" electrons (shown as bright green spheres) out of the metal atoms and the electrons are liberated from the surface of the metal. This is called the photoelectric effect. This effect provided evidence for the discontinuous (discrete) nature of light as distinct from the continuous (wave) nature of light (the quantum theory).
WHAT IS HAPPENING?: Light behaves (in some cases) like particles (photons) little packets (or quanta) of energy. Light rays can be thought of as trains of photons. When a photon of sufficient energy strikes a metal atom an outer electron can absorb the energy from the photon and be ejected ( a process called photoemission). The liberated electron is then called a photoelectron. Some energy is used in liberating the photon (the work function or Φ) and the rest appears as kinetic energy (energy of movement) that gives the electron its momentum. Photons with too little energy (equivalent to light of too long a wavelength) fail to liberate electrons no matter how many of them strike the metal. The photoelectric effect thus has a threshold that varies from one material to another (in some metals it takes less energy to liberate an electron). If the electrons can be attracted to an anode (positive plate) then the photoelectric effect can cause a current to flow. The photoelectric effect finds applications in solar energy generation (solar cells) and in the amplification of incident light (a photomultiplier e.g. in night vision equipment). When current is caused to flow in a solar cell the effect is called the photovoltaic effect and it generally happens in a semiconductor. This ability to create an electric current when light falls on a surface means that detectors can be built that respond when light lands (or fails to land) on a detector. An example would be an intruder alarm that was activated when a beam of light was interrupted.
BRIEF HISTORY: the photoelectric effect was first noticed by Alexandre-Edmond Becquerel who created an actinometer. It was finally explained by Albert Einstein who won the Nobel Prize in Physics (in 1921) for his work.