What Is a Photon? Definition and Facts

What Is a Photon
A photon is a unit of light. It is a quantum or packet with no rest mass, yet with momentum.

A photon is a packet or quantum of light and the force carrier of the electromagnetic force. It is an elementary particle. Like other elementary particles, photons display properties of both particles and waves.

Photon Properties

Photons have the following properties:

  • A photon has zero rest mass. However, because it is moving, it has momentum. So, while packets of light have no mass, they can exert pressure. A photon’s momentum is hν/c, where h is Planck’s constant, ν is the photon’s frequency, and c is the speed of light.
  • A photon has no electrical charge. It is not deflected by an electric or magnetic field.
  • However, photons are affected by gravity.
  • A photon has a spin of 1. Since this is an integer value, a photon is a type of boson.
  • Photons do not obey the Pauli exclusion principle. In other words, more than one photon can occupy a single bound energy state.
  • Photons are stable particles. They do not decay.
  • Photons travel at the speed of light. In a vacuum, this is 299,792,458 meters per second. In a medium, the speed of light depends on the material’s index of refraction.
  • All photons having the same frequency or wavelength have the same energy.
  • Photon energy ranges from radio waves to gamma rays.
  • In a particle-photon interaction, total energy and total momentum are conserved.

Word Origin

The name “photon” comes from the Greek word for light, phôs. Gilbert Newton Lewis coined the term in his December 1926 letter to Nature. However, it had been used by physicists and physiologists prior to this date, mainly referring to the illumination of the eye. Arthur Compton popularized the term in his work, giving Lewis credit for the word.

Photon Symbol

The Greek letter gamma (γ) is the symbol for the photon, probably deriving from work on gamma rays, which were discovered by Paul Villard in 1900. Gamma decay releases photons. The symbol refers to photon energy, where h is Planck’s constant and the Greek letter nu (ν) is the photon frequency. Another symbol is hf, where f is the photon frequency.


The concept of the photon arose from Albert Einstein’s proposed explanation for the photoelectric effect in 1905. The photoelectric effect is the emission of electrons when light strikes a material. Einstein said that the effect was explainable, providing light behaved as a group of discrete (quantized) energy packets rather than solely as a wave. It was Max Planck who had forwarded the idea of light consisting of these quanta. The energy packets became known as photons. Meanwhile, experiments verified Einstein’s explanation.

How Are Photons Produced?

Photons arise as a result of both spontaneous and stimulated emission. Some types of radioactive decay (e.g., gamma and beta decay) release photons, as do particle interactions. Accelerating a charged particle causes photon emission as synchrotron radiation. The annihilation of a particle and its antiparticle (e.g., an electron and positron) results in photon emission. But, mostly the release of photons occurs when electrons transition from excited energy states to more stable ones.

How to Calculate the Energy of a Photon

There are two main equations for calculating the energy of a photon:

E =

Here, E is the photon energy, h is Planck’s constant, and ν is the photon frequency.

E = hc / λ

Here, E is photon energy, h is Planck’s constant, c is the speed of light, and λ is the photon wavelength.


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