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Cherenkov radiation (also spelled Cerenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than that of light in the medium. The characteristic "blue glow" of nuclear reactors is due to Cherenkov radiation. It is named for Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to rigorously characterize it.1 Physical origin
While relativity holds that the speed of light in a vacuum is a universal constant (c), the speed of light in a material may be significantly less than c. For example, the speed of light in water is only 0.75c. Matter can be accelerated beyond this speed during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electronThe electron (also called negatron commonly represented as e&minus is a subatomic particle. In an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. Electrons have the smallest electrical charge and when they mov, exceeds the speed of light in a dielectricMost generally, a dielectric is an insulator, a substance that is highly resistant to flow of electric current. Layers of such substances are commonly inserted into capacitors to improve their performance, and the term dielectric refers specifically to th medium through which it passes.
Moreover, the velocity of light that must be exceeded is the phase velocityThe phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the velocity at which the phase of any one frequency component of the wave will propagate. You could pick one particular phase of the wave (for example th rather than the group velocityThe group velocity of a wave is the velocity with which the overall shape of the wave's amplitude (known as the envelope of the wave) propagates through space. The group velocity is defined in terms of the wave's angular frequency omega and wave number k. The phase velocity can be altered dramatically by employing a periodic medium, and in that case one can even achieve Cherenkov radiation with no minimum particle velocity — a phenomenon known as the Smith-Purcell effectThe Smith-Purcell effect was the precursor of the free electron laser (FEL). In their experiment, Steve Smith, as a graduate student guided by Purcell, sent an energetic beam of electrons very closely parallel to the surface of a ruled optical diffraction. In a more complex periodic medium, such as a photonic crystalPhotonic crystals are periodic dielectric or metallo-dielectric (nano)structures that are designed to affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by, one can also obtain a variety of other anomalous Cherenkov effects, such as radiation in a backwards direction (whereas ordinary Cherenkov radiation forms an acute angle with the particle velocity).
As a charged particle travels, it disrupts the local electromagnetic fieldThe electromagnetic field EMF is composed of two related vectorial fields, the electric field and the magnetic field. This means that the vectors E and B that characterize the field each have a value defined at each point of space and time. If only E the in its medium. Electrons in the atomFor alternative meanings see atom (disambiguation). An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles: electrons, which have a negative charge; protons, which have a positive chs of the medium will be displaced and polarized by the passing EM field of a charged particle. Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed. (In a conductor, the EM disruption can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, when the disruption travels faster than the photons themselves travel, the photons constructively interfere and intensify the observed radiation.
A common analogy is the sonic boom of a supersonic aircraft or bullet. The sound waves generated by the supersonic body do not move fast enough to get out of the way of the body itself. Hence, the waves "stack up" and form a shock front. Similarly, a speed boat generates a large bow shock because it travels faster than waves can move on the surface of the water.
In the same way, a superluminal charged particle generates a photonic shockwave as it travels through an insulator.
In the figure, v is the velocity of the particle (red arrow), β is v/ c, n is the refractive index of the medium. The blue arrows are photons.
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