pho·​ton | \ ˈfō-ˌtän How to pronounce photon (audio) \

Definition of photon

1 : a quantum of electromagnetic radiation Should a substance happen to have a lot of electrons in a higher level, and a lower level is mostly empty …, then a photon can cause an electron to transfer from a higher state to a lower one. This change releases energy and creates a new photon, in addition to the one which caused the transfer. This photon can in turn induce more electrons to fall to a lower state.— Robert Gilmore
2 dated : troland

Keep scrolling for more

Other Words from photon

photonic \ fō-​ˈtä-​nik How to pronounce photon (audio) \ adjective

Science and the Photon

It was Albert Einstein who first theorized that the energy in a light beam exists in small bits or particles, and scientists today know that light sometimes behaves like a wave (somewhat like sound or water) and sometimes like a stream of particles. The energies of photons range from high-energy gamma rays and X-rays down to low-energy infrared and radio waves, though all travel at the same speed. The amazing power of lasers is the result of a concentration of photons that have been made to travel together in order to hit their target at the same time.

Examples of photon in a Sentence

Recent Examples on the Web Unlike both of these options, an optical quantum computer could be a (large) chip-scale device that is powered by an array of laser diodes, with readout done by a series of single-photon detectors. Chris Lee, Ars Technica, "Quantum device performs 2.6 billion years of computation in 4 minutes," 7 Dec. 2020 In the bending of light problem, the sun was modeled identically, but the light was modeled as a massless point traveling at the speed of light (a photon). Tim Childers, Popular Mechanics, "New Theory Casually Upends Space and Time," 10 Dec. 2020 If the light intensity is low enough that only a single photon is present, it is either reflected or transmitted with the same randomness as a fair coin toss. Chris Lee, Ars Technica, "Quantum device performs 2.6 billion years of computation in 4 minutes," 7 Dec. 2020 That’s partly because qubits are highly sensitive, even to the mildest disturbances, like a glancing photon. Robert Hackett, Fortune, "Quantum computing is entering a new dimension," 3 Dec. 2020 The proof-of-principle experiment starts with a photon in each lab. Quanta Magazine, "A New Theorem Maps Out the Limits of Quantum Physics," 3 Dec. 2020 Each photon read out at the end of the process is equivalent to a qubit, revealing the result of a calculation. Tom Simonite, Wired, "China Stakes Its Claim to Quantum Supremacy," 3 Dec. 2020 Electrons injected into the structure cascade down hundreds of energy steps, shedding a photon at each one. Robert F. Service, Science | AAAS, "New lasers that fire terahertz beams could propel medical imaging and contraband detection," 2 Nov. 2020 When a particle, WIMP or otherwise, moves through the tank and hits the nucleus of a xenon atom, the result is energy, in the form of light: a photon, or more likely many. Ryan Bradley, Popular Science, "Glimpse the gold mine where scientists are searching for dark matter," 27 Oct. 2020

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'photon.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of photon

1916, in the meaning defined at sense 2

History and Etymology for photon

phot- + -on entry 2

Keep scrolling for more

Learn More about photon

Statistics for photon

Last Updated

10 Jan 2021

Cite this Entry

“Photon.” Dictionary, Merriam-Webster, Accessed 3 Mar. 2021.

Style: MLA
MLA Chicago APA Merriam-Webster

Keep scrolling for more

More Definitions for photon



English Language Learners Definition of photon

physics : a tiny particle of light or electromagnetic radiation


pho·​ton | \ ˈfō-ˌtän How to pronounce photon (audio) \

Medical Definition of photon

1 : a unit of intensity of light at the retina equal to the illumination received per square millimeter of a pupillary area from a surface having a brightness of one candela per square meter

called also troland

2 : a quantum of electromagnetic radiation

More from Merriam-Webster on photon Encyclopedia article about photon

Comments on photon

What made you want to look up photon? Please tell us where you read or heard it (including the quote, if possible).


Test Your Vocabulary

February 2021 Words of the Day Quiz

  • squirrel in winter
  • Which is a synonym of perdure?
Name That Thing

Test your visual vocabulary with our 10-question challenge!

 AlphaBear 2

Spell words. Make bears.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!