phosphorescence

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phosphorescence, a faint blue-green light emitted from the sea at night, either as a continuous glow or a series of tiny flashes. This light is produced by the bioluminescence of living organisms ranging from bacteria to the many species of plankton, including phytoplankton, especially dinoflagellates. On moonless nights in late summer, flashes of light in the surf breaking on sandy beaches are often produced by the appropriately named dinoflagellate Noctiluca. Bioluminescence is produced by a chemical reaction and serves a wide range of functions. Deep in the ocean where there is little or no daylight, almost every species has some light-producing capability. The wavelength of the blue-green light emitted is the same as the residual daylight that penetrates to the greatest depths in the ocean. In a darkened environment signalling by light is an effective way of communicating with other animals of the same species, especially to confirm species identification during mating, or to indicate a readiness to mate. Lights are also used on lures to tempt prey within range. Bright flashes are used as a defence mechanism, startling the attacker. Glowing smoke screens or even phantom decoys are also used to avoid attack. In an environment where the brightest daylight comes from directly overhead, animals are particularly vulnerable to attack from below by predators that pick out their silhouettes. Lights arranged along the underside of fish and some Crustacea like prawns break up the silhouette, and act as a form of camouflage.

Some phosphorescent phenomena have a physical origin. During electrical storms structures like a ship's mast can become surrounded by a corona of electrical discharges giving an eerie bluish light called St Elmo's Fire.

Bibliography

Herring, P. , The Biology of the Deep Ocean (2002).

M. V. Angel

phosphorescence , luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescence in that it continues even after the radiation causing it has ceased. Phosphorescence was first observed in the 17th cent. but was not studied scientifically until the 19th cent. According to the theory first advanced by Philipp Lenard, energy is absorbed by a phosphorescent substance, causing some of the electrons of the crystal to be displaced. These electrons become trapped in potential troughs from which they are eventually freed by temperature-related energy fluctuations within the crystal. As they fall back to their original energy levels, they release their excess energy in the form of light. Impurities in the crystal can play an important role, some serving as activators or coactivators, others as sensitizers, and still others as inhibitors, of phosphorescence. Organo-phosphors are organic dyes that fluoresce in liquid solution and phosphoresce in solid solution or when adsorbed on gels. Their phosphorescence, however, is not temperature-related, as ordinary phosphorescence is, and some consider it instead to be a type of fluorescence that dies slowy.

phosphorescence The property of some minerals of emitting light during exposure to X-rays, ultraviolet light, or cathode radiation and continuing to do so after the exposure has ceased (if light emission ends when the radiation source is switched off, the property is called ‘fluorescence’). The colour of the emitted light varies with the wavelength of the radiation to which the mineral is exposed, and is thought to be due to the presence of traces of organic material or cations within the atomic structure of the mineral.

phos·pho·res·cence / ˌfäsfəˈresəns/ • n. light emitted by a substance without combustion or perceptible heat: the stones overhead gleamed with phosphorescence. ∎ Physics the emission of radiation in a similar manner to fluorescence but on a longer timescale, so that emission continues after excitation ceases. DERIVATIVES: phos·pho·res·cent adj.

phosphorescence Form of luminescence in which a substance emits light of one wavelength. Unlike fluorescence, it may persist for some time after the initial excitation. In biology, phosphorescence is the production of light by an organism without associated heat, as with a firefly. In warm climates, the sea often appears phosphorescent at night, as a result of the activities of millions of microscopic algae.

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