Electromagnetic radiation refers to a form of linear energy transfer. Visible light is electromagnetic radiation, but is only a small slice of the broad electromagnetic spectrum. The propagation of this radiation, of one or more particles, gives rise to many phenomena such as attenuation, absorption, diffraction and refraction, redshift, interference, echoes, electromagnetic interference and biological effects.
Electromagnetic radiation can be described in a corpuscular way as the propagation of photons (boson vector of the electromagnetic interaction), or in a wave way as an electromagnetic wave. It manifests itself in the form of an electric field coupled to a magnetic field.
The electromagnetic wave and the photon
Light refers to electromagnetic radiation visible to the human eye. Radio waves, X rays and γ rays are also electromagnetic radiation.
Due to the wave-particle duality, electromagnetic radiation can be modeled in two complementary ways:
- electromagnetic wave: radiation is the propagation of a variation in electric and magnetic fields; a spectrograph allows this wave to be broken down into monochromatic waves of wavelengths λ and different frequencies ν which can then be analyzed;
- photon: quantum mechanics associates the normal modes of monochromatic electromagnetic radiation with a particle of zero mass and spin 1 called photon whose energy is E = hν where h is Planck’s constant.
The pulse p of the photon is equal to p = E/c = hν/c.
The energy of the photons of an electromagnetic wave is conserved when passing through various transparent media (on the other hand, a certain proportion of photons can be absorbed).
In a vacuum, electromagnetic radiation, and in particular light, travels at a speed of 299,792,458 m/s. This speed, called the speed of light and noted c, is one of the fundamental physical constants.
The wavelength is equal to:
λ = cν/ν
cν being the speed of light in the medium considered for the frequency ν, with cν = c/nν (nν being the refractive index of monochromatic light of frequency ν in the medium considered).
The observation, at the end of the 19th century, that the speed of light in a vacuum does not depend on the frame of reference, led to the development of the special theory of relativity.
- Any body at a temperature above absolute zero, either -273.15 °C or 0 K or -459.67 °F emits electromagnetic radiation called thermal radiation or black body radiation.
- A body that receives electromagnetic radiation can reflect some of it and absorb the rest. The absorbed energy is converted into thermal energy and contributes to the increase of the temperature of this body.
- A charged particle with high energy emits electromagnetic radiation:
- when it is deflected by a magnetic field: it is synchrotron radiation; this synchrotron radiation is used as an X-ray source for many physics and biology experiments (lines of light around a synchrotron);
- when it enters a different environment: it is the “continuous braking radiation”;
- The absorption of a photon can cause atomic transitions, that is to say, to excite an atom whose energy increases by the modification of the orbital of one of its electrons.
- When an excited atom returns to its fundamental energy state, it emits a photon whose energy (and therefore frequency) corresponds to a difference between two energy states of the atom.
- Some electromagnetic radiation carries enough energy to be able to extract electrons from matter, which is then ionizing radiation.
- In the same area of the electromagnetic spectrum, photons are capable of forming electron-hole pairs in semiconductors. By recombining, the electron and the hole emit light (principle of diodes).
- Nuclear reactions, such as those of fission, fusion and decay, are often accompanied by the emission of high-energy photons called γ rays (gamma rays).
An electromagnetic spectrum is the decomposition of electromagnetic radiation according to its wavelength, or, equivalently, its frequency (via the propagation equation) or the energy of its photons.
(Electromagnetic spectrum with visible light highlighted. )
For historical reasons, electromagnetic waves are referred to by different terms, depending on the frequency (or wavelength) ranges. By decreasing wavelength, they are:
- radio waves and radar waves are produced by low frequency electric currents;
- infrared waves, visible light and ultraviolet radiation are produced by electronic transitions in atoms, involving peripheral electrons, as well as by thermal radiation; ultraviolet waves have effects on the skin (tanning, sunburn, skin cancer);
- X-rays can also be produced during high energy electronic transitions. They are, for example, generated by radioactivity (fluorescence photons emitted during the reorganization of the electronic procession of an atom). Their controlled generation is most often carried out by electron braking (X-ray tube) or by synchrotron radiation (deflection of relativistic electron beam). Due to their sub-nanometric wavelength, they allow the study of crystals and molecules by diffraction; hard X-rays correspond to photons of higher energy, and soft X-rays to photons of lower energy;
- γ radiation is produced by radioactivity during the de-excitation of a nucleus. They are therefore emitted in particular by radioactive materials and nuclear reactors. Their energy is therefore on average higher than X photons.