photoelectric effect

Process by which electromagnetic radiation, including visible light, incident on a material releases an electric charge. It is commonly thought of as the emission of electrons from a substance (usually a metallic surface) when it is struck by photons (quanta of electromagnetic radiation), usually those of visible light or ultraviolet radiation.

The energy of the emitted electrons depends on the frequency of the incident radiation (which must exceed a characteristic threshold frequency); the higher the frequency, the greater the energy of its photons. The number of electrons emitted depends on the radiation's intensity (rate of transfer of energy per unit area). The effect was first observed 1887 by Heinrich Hertz (better known for his researches on electromagnetic waves), who showed that electric sparks occur more easily when the electrodes are illuminated with ultraviolet light. It was established in 1900 as being due to the ejection of electrons by the light. This discovery, along with the identification of the beta particles observed in radioactivity as being electrons also, helped establish the fundamental nature of electrons as a constituent of all atoms. The theory of the photoelectric effect, a quantum theory of radiation, was formulated by Albert Einstein 1905.