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Photons



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 that Max Planck and Albert Einstein referred to as a ‘quantum’ of energy when they first established the particle nature of light.

The photon : an elementary wave
a fundamental wave: A photon is a microscopic electromagnetic wave, caused by oscillations in the electric and magnetic fields through space and time. A wave, in the classical sense of the term, consists of infinite oscillations and can never be localised. The number of oscillations in the photons emitted by atoms and nuclei is not infinite, thanks to a small uncertainty in their energy values. The shorter the wavelength of the photons, the denser the wave is in space. When the wavelength is as short as is the case with gamma rays, then the waves become dense enough to resemble particles.
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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.

Three types of photons
The photon energy varies inversely with its wavelength. Photons are usually emitted by molecules, with energies of a few electronvolts and wavelengths of a fraction of a micron (a thousandth of millimetre). X-rays are emitted by the deeper layers of the atom, with energies reaching hundreds of thousands of electronvolts (keV) and wavelengths of the order of a billionth of a metre (nanometres). Gamma rays, on the other hand, are emitted by nuclei and have even higher energies with wavelengths appropriate to the dimensions of a nucleus.
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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).

From infrared to gamma rays
This diagram shows the same celestial object – the Crab nebula – as seen by different types of photons: infrared, visible light, X-rays and gamma rays. All these photons have the same nature, though their wavelengths (which define the colour in the visible part of the spectrum) and energies are different. The image taken with high-energy gamma rays was obtained thanks to the 4-telescope setup of the HESS experiment, used to identify and measure the direction of high-energy gamma rays.
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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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