What is a Photon? The Best Explanatory Guide

Compiled by Stanley Udegbunam || Dec 08, 2020

define photon



A photon is the smallest discrete amount of electromagnetic radiation.

In physics, the smallest possible unit of energy or any substance at the sub-atomic level is called a quantum.

Photons can be defined as a quantum of light or a quantum of electromagnetic radiation.

The plural of quantum is quanta. 

Imagine beams of sunlight radiating through your window on a hot afternoon.

According to quantum physics, a light beam is made of zillions of tiny packets of light, streaming through the air.

This tiny packet of light or energy is nothing but photons.

In other words, what we see as light is just tiny bundles of photons.

Photon is the basic unit of every light and always in motion.

They also travel at the speed of light, c = 2.998 x 108 m/s.

photons - beam of light

Beams of sunlight radiating through a window


Gilbert N. Lewis popularized the term photon for energy units.

Prior to this, Einstein has earlier published a paper where he modelled electromagnetic wave as light quantum using his photoelectric effect experiment.

Einstein’s light quantum theory was built upon the research of Max Planck on black body radiation from way back in 1900.

Nevertheless, it was Lewis who brought the term “photon” into common usage through a letter sent to nature scientific journal on December 18, 1926.

We can say that the term “photon” was birthed from the works of Gilbert N. Lewis, Albert Einstein and Max Planck.

In Greek, photon means light.

photon - light rays from window


Since photons are the basic unit of light, they exhibit wave-particle duality meaning they have properties of both waves and particles.

  1. They are stable and always in motion.
  2. In a vacuum, they move at a constant speed equivalent to the speed of light c= 2.998 x 108 m/s.
  3. They have zero mass and rest energy. They only exist as moving particles.
  4. Photons are electrically neutral. They have no electric charge.
  5. They are spin-1 particles which makes them bosons with a spin axis that is parallel to the direction of travel. This feature is what allows for polarization of light.
  6. They carry energy and momentum which are dependent on the frequency.
  7. They can have interactions (i.e. collisions) with other particles or electrons, such as the Compton effect in which particles of light collide with atoms, causing the release of electrons.
  8. They can be destroyed or created when radiation is absorbed or emitted.
photon of light


There are some photon facts that though, may look strange; they are actually worth knowing.

  • The photon is an elementary particle, despite the fact that it has no mass.
  • Photon cannot decay on its own, although the energy of the photon can transfer/created upon interaction with other particles.
  • Not only is light made up of photons, but all electromagnetic energy (i.e. microwaves, radio waves, X-rays) is made up of photons.
  • Photons are their own anti-particle. Meaning, Photon, and Anti-photon are nothing but the same particle.
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