Luminescence

Luminescence, emission of light by means other than combustion and therefore occurring at lower temperatures than are required for combustion. An example of luminescence is the light, or glow, emitted by a luminous watch dial. Luminescence contrasts with incandescence, which is the production of light by heated materials.

When certain materials absorb various kinds of energy, some of the energy may be emitted as light. This process involves two steps: (1) the incidental energy causes the electrons of the atoms of the absorbing material to become excited and jump from the inner orbits of the atoms to the outer orbits; (2) when the electrons fall back to their original state, a photon of light is emitted. The interval between the two steps may be short (less than 1/100,000 of a sec) or long (many hours). If the interval is short, the process is called fluorescence; if the interval is long, the process is called phosphorescence. In either case the light produced is almost always of lesser energy, that is, of longer wavelength, than the exciting light. Fluorescence and phosphorescence have a number of practical applications. The picture screens in television receivers are coated with fluorescent materials known as phosphors that glow when excited by a cathode ray (see Cathode-Ray Tube). The interiors of fluorescent lamps have similar coatings, which absorb the invisible but intense ultraviolet components of the primary light source and emit visible light. A special type of fluorescence called stimulated emission occurs in the operation of a laser.

Chemiluminescence is caused by chemical reaction, as when yellow phosphorus oxidizes in air, emitting green luminescence. If the chemical reaction takes place in a living organism, such as the firefly, the process is called bioluminescence. Triboluminescence is luminescence that results from the breaking, scratching, or pulling apart of certain materials. Electroluminescence is luminescence created by a gas in the path of an electrical discharge; examples are lightning and the light of a fluorescent lamp. Photoluminescence is luminescence created when certain materials are irradiated by visible light or ultraviolet light; an example is the phosphorescence of paints. Roentgenoluminescence is luminescence produced by X rays bombarding certain materials; an example is X rays on a fluoroscope screen. Sonoluminescence, observed in some organic liquids, is luminescence produced by ultrahigh-frequency sound waves, or ultrasound.

See also Light; Lightning; Quantum Theory; Radiation.