Photon Definition

Light beams reflecting
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By
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Anne Marie Helmenstine, Ph.D.
Updated February 01, 2019

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)

Characteristics

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 velocity, c = 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.