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What Is a Photon? Properties of Photons

What is a photon? Properties of photons

The photon is an elementary particle that carries all forms of electromagnetic radiation. This photon definition includes X-rays, gamma rays, a visible beam of light, infrared light, radio waves, and microwaves.

Specifically, the definition of a photon (represented by the symbol γ) tells us, "A photon is the quantum of energy in the form of electromagnetic radiation, emitted or absorbed by matter.”

Photons are emitted in many natural processes, such as:

  • When a charge speeds up, it emits synchrotron radiation.

  • During a molecular, atomic, or nuclear transition to a lower energy level, photons of various energies will be emitted, ranging from radio waves to gamma rays.

  • When a particle and its corresponding antiparticle annihilate.

In the standard model of particles, photons and other elementary particles are described as a necessary consequence of the fact that the principles of physics have a certain symmetry in space-time.

What Is the Energy of a Photon?

The photon energy corresponds to the energy carried by a single photon in an electric or magnetic field. This energy depends on the electromagnetic frequency and wavelength. Consequently, the energy increases with the increasing frequency and decreasing the photon wavelength.

The most commonly used units to indicate the energy of these particles are the electron volt (eV) and the joule (J) - one joule is equivalent to 6.24 × 10 18 eV. 

Properties of a Photon

It has no electric charge or rest mass and is a stable particle. 

When they are in a vacuum, photons travel at the speed of light and have two possible polarization states.

The photon is the gauge boson for electromagnetism. Therefore, all the other quantum numbers in the photon (such as the number of leptons, baryons, and flavor quantum numbers) are zero. Furthermore, the photon does not obey the Pauli exclusion principle but obeys the Bose-Einstein statistics.

The energy and momentum of a photon depend only on its frequency or, inversely, its wavelength. Therefore, high energy photons mean the photons whose frequency is very high.

Photons present wave-particle duality, meaning that photons act as a wave and as particles simultaneously.

Many natural processes emit photons, such as:

  • When a charge accelerates, it emits synchrotron radiation.

  • During a molecular, atomic, or nuclear transition to a lower energy level, photons of various energies will be emitted, ranging from radio waves to gamma rays.

  • When a particle and its corresponding antiparticle are annihilated (for example, electron-positron annihilation).


Elementary particle


Electromagnetic, Weak, Gravity




Albert Einstein (1905)

The name "photon" is generally attributed to Gilbert N. Lewis (1926)



<1 × 10 −18  eV / c 2

Average life


Electric charge

0 <1 × 10 −35  e 





C parity



Applications and Uses of Photons

These fundamental particles have many applications in technology. Some notable examples are:

  • Obtaining electrical energy: the impact of a photon on the surface of a solar panel causes the release of electrons from the semiconductor materials due to the photovoltaic effect. The sum of this movement of charges gives rise to a direct current.

  • Chip manufacturing uses this type of particle.

  • Engineering and chemistry: They are used to calculate the change in energy resulting from photon absorption and determine the frequency of light emitted by given photon emission.

  • Molecular biology: In some cases, two energy transitions can be coupled so that as one system absorbs a photon, another nearby system "steals" its energy and re-emits a photon of a different frequency.

  • Random number generation: Several types of hardware random number generators involve the detection of single photons.

When Did the Concept of Photon First Appear?

In most theories up to the 17th and 18th centuries, the light was considered to be made up of particles. However, because particle models could not explain certain phenomena, some scientists proposed wave theories for light.

Albert Einstein gradually developed the modern concept of the photon in the early 20th century. This concept was used to explain experimental observations that did not agree with the classical model of light as an electromagnetic wave.

Photons, also called light quantum, are minute energy packets of electromagnetic radiation. The concept originated in Einstein's explanation of the photoelectric effect, in which he proposed the existence of discrete energy packets during light transmission.

In Max Planck's model, light was described by Maxwell's equations, but material objects that emitted and absorbed light did so in discrete packets of energy. Although these semi-classical models contributed to the development of quantum mechanics, several subsequent experiments validate Einstein's hypothesis that light itself is quantized.

In 1926 the optical physicist Frithiof Wolfers and the chemist Gilbert N. Lewis coined the term "photon" for these particles.




    Published: May 13, 2015
    Last review: August 4, 2022