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

The result is a hybrid material that is both robust and highly efficient at emitting photons. —Christopher McFadden, Interesting Engineering, 27 Sep. 2025

The magnetic and laser forces are arranged so that any lithium atom that might otherwise fly away from the center is most likely to absorb a photon from the lasers, getting a momentum kick back into the trap. —IEEE Spectrum, 24 Sep. 2025

As the Universe expands, the wavelength of each photon gets lengthened by the expansion, meaning that the amount of energy present in the form of radiation decreases faster than the amount of energy present in the form of matter in the expanding Universe. —Ethan Siegel, Big Think, 23 Sep. 2025

The system encodes data into pulses of laser light, transforming digital information into streams of photons that can travel millions of miles — a significant shift from the radio frequencies used by NASA's Deep Space Network for decades. —Samantha Mathewson, Space.com, 23 Sep. 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