The elementary components of light and electromagnetic waves
Light is composed of infinitesimal individual electromagnetic waves known as photons. Each photon carries a minuscule amount of energy. Max Planck and Albert Einstein referred them as a ‘quantum’ of energy when they first established the particle nature of light.
Despite what the Greek name would suggest, photons do not carry always light. Light waves do have the remarkable property of leaving an impression on our retinas, but many other photons exist that we do not see.
One groups together under the name ‘photon’ a variety of electromagnetic rays, from radio waves to X Rays and gamma rays, in addition to 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 and a 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 core of Einstein’s theory of relativity.The relativity of time has no effect on our daily lives. In nuclear and particle physics, however, where scientists study particles whose speed approaches the speed of light, relativistic effects are very common.
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 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 compact and capable of easily passing through atoms, and as a result are highly penetrative.
Gamma rays emitted by nuclei have energies 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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