A photon is a particle of light, defined as a quantum of electromagnetic (or light) energy. Photons travel at the speed of light in a vacuum (more commonly referred to as simply the speed of light) c = 300,000 km/s. Photons are constantly in motion, and vacuum (absolutely empty space) has a constant light speed for all observers.

It is an elementary particle or also a quantum of light. Photons can be absorbed or emitted by atoms or molecules. Once a photon is absorbed, the energy is transferred to the atom or molecule. Because the energy is quantized, all of the photon’s energy is transferred.

The same is true for the reverse process. When an atom or molecule loses energy, it emits a photon that contains energy that exactly matches the energy lost by the atom or molecule. The change in energy is directly proportional to the frequency of the emitted or absorbed photon.

Photons are electrically neutral. A photon is an elementary particle, despite its having no mass. It cannot decay independently, although photon energy can be transferred (or created) by interacting with other particles.

Photons have the properties of both waves and particles. This is outside of how we usually perceive things. Billiard balls act like particles, and we see waves in water. One consequence of this wave-particle duality is that photons, although considered as particles, can have frequency, wavelength, amplitude, and other properties inherent in wave mechanics.

Electromagnetic waves can be divided and sorted according to different wavelengths/frequencies; this sorting is called the electromagnetic spectrum. The following picture shows this spectrum. All types of electromagnetic radiation that occur in the Universe are included.

The electromagnetic spectrum consists of all the different types of radiation in the Universe. Gamma rays have the highest frequency, while radio waves have the lowest. Visible light is roughly in the middle of this spectrum and covers only a very small part of it.





What you should know about photons:

  • photons behave like a particle and a wave at the same time and always move at the speed of light;

  • photons have zero mass;

  • photons carry energy, information and momentum;

  • photons can be absorbed and generated;

  • photons interact with other particles. For example, they can knock electrons out of an atom, giving them the energy they need to get out when they collide;