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[foh-ton] /ˈfoʊ tɒn/
a quantum of electromagnetic radiation, usually considered as an elementary particle that is its own antiparticle and that has zero rest mass and charge and a spin of one. Symbol: γ.
Also called light quantum.
Origin of photon
1900-05; phot- + -on1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for photon
  • When a photon strikes its surface, it bounces off, imparting its momentum to the sail.
  • When the atom drops back to a lower energy level, it emits a photon.
  • Quantum coherence describes how more than one molecule interacts with the same energy from one incoming photon at the same time.
  • Once entangled, a photon can carry any information stored in the atom's quantum state to other parts of the computer.
  • And the opposite in the case of a photon traveling away from a source of gravity.
  • If these particles zip through water or ice, they leave faint blue trails of light that can be seen by sensitive photon detectors.
  • Not a rocket with its boosters on full blast nor a photon of light.
  • The material included a layer of equally spaced quantum dots, which emit lots of light when struck with one photon.
  • When they fall back to a lower state a photon is emitted, and the wavelength of the photon determines the color.
  • Each photon of laser light carries a tiny amount of momentum.
British Dictionary definitions for photon


a quantum of electromagnetic radiation, regarded as a particle with zero rest mass and charge, unit spin, and energy equal to the product of the frequency of the radiation and the Planck constant
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for photon

"unit of electromagnetic radiation," 1926 in modern sense, from photo- "light" + -on "unit."

Online Etymology Dictionary, © 2010 Douglas Harper
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photon in Medicine

photon pho·ton (fō'tŏn')
The quantum of electromagnetic energy, generally regarded as a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime.

pho·ton'ic adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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photon in Science
The subatomic particle that carries the electromagnetic force and is the quantum of electromagnetic radiation. The photon has a rest mass of zero, but has measurable momentum, exhibits deflection by a gravitational field, and can exert a force. It has no electric charge, has an indefinitely long lifetime, and is its own antiparticle. See Note at electromagnetic radiation. See Table at subatomic particle.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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photon in Culture
photon [(foh-ton)]

The quantum, or bundle of energy, in which light and other forms of electromagnetic radiation are emitted. (See atom.)

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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