fotoelectric

The fotoelectric effect, also known as the photoelectric effect, is the emission of electrons from a material, typically a metal, when it is illuminated by light. In quantum terms, light is viewed as consisting of photons, each with energy hf. An electron is ejected only if a photon's energy exceeds the material’s work function φ; the surplus energy becomes the kinetic energy of the emitted electron, given by KEmax = hf − φ. The emission rate increases with light intensity (provided hf > φ), while the kinetic energy of the emitted electrons depends mainly on the photon energy rather than the light’s intensity. A threshold frequency exists below which no emission occurs, and at high light intensities multi-photon processes can contribute.

History and significance: The effect was first observed by Heinrich Hertz in the late 19th century and was given a quantum explanation by Albert Einstein in 1905, introducing the concept of photons. Experiments by Philipp Lenard demonstrated electron emission from metal surfaces and established the relation between stopping potential and photon energy. Robert Millikan’s measurements further confirmed KEmax = hf − φ and helped determine work function values for various materials.

Applications and related concepts: The fotoelectric effect underpins photoelectron spectroscopy, including ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), which probe electronic structure and binding energies. It also informs the design of photomultiplier tubes and other light-detection devices. The effect differs from the photovoltaic effect, which generates electrical current without electron emission, a mechanism exploited in solar cells. See also: work function, photoemission, photoelectric spectroscopy, photovoltaic effect.