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noun, plural: photons

(physics) A quantum unit of light energy or electromagnetic radiation


A photon is a quantum of radiant energy with a visible wavelength. It is an elementary particle that is its own antiparticle. It is a discrete concentration of energy. It is a massless elementary particle that moves at the speed of light. It has no electric charge 1 and is stable. It is the unit or quantum of electromagnetic radiation or light energy.

Photons are emitted when electrons move from one energy state to another. A photon that is emitted through a biological system is referred to as a biophoton. Fluorescence, for instance, is the absorption and the immediate re-emission of light or photon, which has longer wavelength when re-emitted than that of what was previously absorbed. It occurs as a prior exposure to light and eventually absorbs it then releases it. Similarly, phosphorescence is the emission of light at temperatures below incandescence, as a result of a previous light exposure. The difference is that in phosphorescence, the emission of the photon is delayed. Bioluminescence does emit biophotons. However, it is produced by biochemical means. Therefore, the light emitted is not considered as biophotons.

In terms of photosynthesis, light energy is an important requirement for photosynthesis to occur. The chlorophyll reaction center absorbs photons, and consequently, becomes activated and acts as electron donors.

Symbol: γ


  • light quantum

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Related term(s):

1 Kobychev, V.V. and Popov, S.B. (2005). "Constraints on the photon charge from observations of extragalactic sources". Astronomy Letters. 31 (3): 147–151.