What are photons?

Photons are elementary particles that are regarded as quanta of the electromagnetic field. They are the carriers of the electromagnetic force, one of the four fundamental forces of physics (the other three being gravity, the strong nuclear force and the weak nuclear force).

Photons are unique in that they have no mass and always travel at the speed of light, which is approximately 299,792 kilometres per second in a vacuum. They can have different energies, which determine their wavelength and frequency and, consequently, the type of light they produce. For example, photons with a short wavelength and high frequency have more energy and produce ultraviolet or X-ray light, whilst photons with a long wavelength and low frequency have less energy and produce infrared or radio waves.

Photons play a central role in many areas of physics, including quantum mechanics, quantum electrodynamics and quantum information theory. They are also important for many technological applications, such as laser technology, fibre-optic communications, imaging and solar cell technology, where they convert the energy of sunlight into electrical energy.