photon

noun

pho·​ton ˈfō-ˌtän 

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

fō-ˈtä-nik 

adjective

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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

While a sailboat harnesses the lift created by wind flowing over its curved sails to move across water, a solar sail uses the momentum of photons from sunlight, reflected off its large, shiny sail, to propel a spacecraft through space. —Mojtaba Akhavan-Tafti, Space.com, 3 Aug. 2025

The machine, the Linac Coherent Light Source at the SLAC National Accelerator Laboratory in California, accelerates electrons to more than 99 percent the speed of light and then shoots them through undulating magnetic fields to create a very bright beam of one trillion (1012) x-ray photons. —Clara Moskowitz, Scientific American, 23 July 2025

There’s silver dust in the cells, and some boron and phosphorus, critical additives to increase conductivity and to provide the necessary environment for photons from sunlight to knock electrons loose from the silicon. —Bill McKibben, New Yorker, 9 July 2025

After the photons bounce among Rubin’s mirrors, their final stop before being transformed into data is the world’s largest digital camera. —Michael Jones McKean, The Atlantic, 23 June 2025

Word History

Etymology

phot- + -on entry 2

First Known Use

1916, in the meaning defined at sense 2

Rhymes for photon

• anton
• argon
• axon
• baronne
• baryon
• baton
• boron
• buisson
• bygone
• capon
• cardon
• chiffon

Kids Definition

photon

noun

pho·​ton ˈfō-ˌtän 

: a tiny particle or bundle of electromagnetic radiation

Medical Definition

photon

noun

pho·​ton ˈfō-ˌtän 

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