photon

noun pho·ton \ ˈfō-ˌtän \

Updated on: 17 Apr 2018

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

—

photonic

play \fō-ˈtä-nik\ adjective

See photon defined for English-language learners

See photon defined for kids

Recent Examples of photon from the Web

For several years now, companies like Ribordy’s have sold quantum random number generators based on photons.

— sophia chen, WIRED, "Quantum Mechanics Could Solve Cryptography’s Random Number Problem," 11 Apr. 2018
When excited, an atom can absorb a third photon, which strips away its positron entirely.

— adrian cho, Science | AAAS, "Atoms and antiatoms haven’t crashed Albert Einstein’s theory of relativity—yet," 4 Apr. 2018
But most microwave cavities can only host photons of a single frequency.

— adrian cho, Science | AAAS, "Weird vibrations poised to control quantum computers," 14 Mar. 2018
One possibility is that the force of gravity is carried by a particle, like how electricity is carried by the electron or how light is carried by the photon.

— avery thompson, Popular Mechanics, "Scientists Propose Experiment to Test Quantum Gravity," 8 Mar. 2018
Researchers who want to predict the behavior of systems governed by quantum mechanics—an electron in an atom, say, or a photon of light traveling through space—typically turn to the Schrödinger equation.

— philip ball, Scientific American, "Does a Quantum Equation Govern Some of the Universe's Large Structures?," 7 Mar. 2018
Traditional therapy irradiates tumors with X-ray waves, which are beams of photons, and all tissue along the beams’ paths get a similar dose of radiation.

— bonnie berkowitz, Washington Post, "How proton therapy attacks cancer," 5 Mar. 2018
Already photons speed our communications along fiber optic lines.

— marissa fessenden, Smithsonian, "Scientists Create a New Form of Light by Linking Photons," 16 Feb. 2018
From the pitch-black of a rural Franklin hilltop under the frigid sky, a serene voice breaks the silence, describing how photons from distant stars have traveled light-years through space to fall upon their viewer's eye.

— janice steinhagen, Courant Community, "Franklin Astronomer Brings Night Sky Down To Earth," 13 Feb. 2018

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.

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.

Origin and Etymology of photon

phot- + ²-on

NEW! Time Traveler

First Known Use: 1916

in the meaning defined at sense 2

See Words from the same year

PHOTON Defined for English Language Learners

photon

noun

Definition of photon for English Language Learners

physics : a tiny particle of light or electromagnetic radiation

Medical Dictionary

photon

noun pho·ton \ ˈfō-ˌtän \

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