Light is composed of infinitesimal individual electromagnetic waves known as photons. Each photon carries a minuscule amount of energy that Max Planck and Albert Einstein referred to as a ‘quantum’ of energy when they first established the particle nature of light.
Despite what the Greek root of the word would suggest, photons do not carry only light. Light waves do have the remarkable property of leaving an impression on our retinas, but many other photons exist that we are not capable of seeing.
We group together under the name ‘photon’ a large variety of electromagnetic rays, from radio waves to X-rays and gamma rays, including infrared, visible and ultraviolet light.
Like all waves, light can propagate. The speed of light – and more generally, the speed of electromagnetic waves - in a vacuum is the largest speed known to us and the universal speed limit: it is impossible to travel faster. It is equally impossible to slow a photon down, a fact which physicists have interpreted as indicating that photons have no mass.
The impossibility of travelling faster than the speed of light in vacuum (one of the fundamental constants in physics) is at the heart of Einstein’s theory of relativity.
As quanta of electromagnetic energy, photons are defined by ... their energy. As a fundamental wave, they are also defined by their wavelength. Albert Einstein was the first to show that the product of a photon’s energy by its wavelength is a constant – 1.24 electronvolt microns (if one chooses units of energy and length which are tiny but appropriate to photons of light).
According to this equation, photons with higher energies have shorter wavelengths and vice versa. Photons emitted by nuclei have an energy about a million times higher than that of photons emitted by a molecule or an atom, and were first discovered by Paul Villard in 1899. The tiny wavelengths of these ‘gamma rays’ makes them very dense and capable of easily passing through atoms, and as a result are highly penetrative.
The gamma rays emitted by nuclei have an energy of the order of a million electronvolts (MeV). Particle physics focuses on gamma rays with much higher energies; of the order of several thousand MeV, if not higher.
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