Photon Definition

Light beams reflecting
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Photon Definition: A photon is a discrete packet of energy associated with electromagnetic radiation (light). A photon has energy E which is proportional to the frequency ν of the radiation: E = hν, where h is Planck's constant.

Also Known As: quantum, quanta (plural)


Photons are unique in that they have characteristics of both particles and waves at the same time. For students, it remains unclear as to whether a photon is a particle that travels in a wave pattern or a wave broken up into particles. Most scientists simply accept the photon as a unique packet of energy that has characteristics of both waves and particles.

Properties of a Photon

  • Behaves like a particle and a wave, simultaneously
  • Moves at a constant velocityc = 2.9979 x 108 m/s (i.e. "the speed of light"), in empty space
  • Has zero mass and rest energy
  • Carries energy and momentum, which are also related to the frequency (nu) and wavelength (lamdba) of the electromagnetic wave, as expressed by the equation E = h nu and p = h / lambda.
  • Can be destroyed/created when radiation is absorbed/emitted.
  • Can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect in which particles of light collide with atoms, causing the release of electrons.
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Helmenstine, Anne Marie, Ph.D. "Photon Definition." ThoughtCo, Aug. 27, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 27). Photon Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "Photon Definition." ThoughtCo. (accessed April 21, 2021).