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Cherenkov radiation, also known as Vavilov–Cherenkov radiation, is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to detect it experimentally.〔 Reprinted in Selected Papers of Soviet Physicists, ''Usp. Fiz. Nauk'' 93 (1967) 385. V sbornike: Pavel Alekseyevich Čerenkov: Chelovek i Otkrytie pod redaktsiej A. N. Gorbunova i E. P. Čerenkovoj, M.,"Nauka,'' 1999, s. 149-153. ((ref ))〕 A theory of this effect was later developed within the framework of Einstein's special relativity theory by Igor Tamm and Ilya Frank, who also shared the Nobel Prize. Cherenkov radiation had been theoretically predicted by the English polymath Oliver Heaviside in papers published in 1888–89. == Physical origin == While electrodynamics holds that the speed of light ''in a vacuum'' is a universal constant (''c''), the speed at which light propagates in a material may be significantly less than ''c''. For example, the speed of the propagation of light in water is only 0.75''c''. Matter can be accelerated beyond this speed (although still to less than ''c'') during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light would otherwise propagate in the same medium. Moreover, the velocity that must be exceeded is the phase velocity of light rather than the group velocity of light. 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 effect. In a more complex periodic medium, such as a photonic crystal, 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 field in its medium. In particular, the medium becomes electrically polarized by the particle's electric field. If the particle travels slowly then the disturbance elastically relaxes back to mechanical equilibrium as the particle passes. When the particle is traveling fast enough, however, the limited response speed of the medium means that a disturbance is left in the wake of the particle, and the energy contained in this disturbance radiates as a coherent shockwave. A common analogy is the sonic boom of a supersonic aircraft or bullet. The sound waves generated by the supersonic body propagate at the speed of sound itself; as such, the waves travel slower than the speeding object and cannot propagate forward from the body, instead forming a shock front. In a similar way, a charged particle can generate a light shock wave as it travels through an insulator. In the figure, the particle (red arrow) travels in a medium with speed such that , where is speed of light in vacuum, and is the refractive index of the medium. (If the medium is water, the condition is , since for water at 20 °C.) We define the ratio between the speed of the particle and the speed of light as . The emitted light waves (blue arrows) travel at speed . The left corner of the triangle represents the location of the superluminal particle at some initial moment (''t''=0). The right corner of the triangle is the location of the particle at some later time t. In the given time ''t'', the particle travels the distance : whereas the emitted electromagnetic waves are constricted to travel the distance : So: : Note that since this ratio is independent of time, one can take arbitrary times and achieve similar triangles. The angle stays the same, meaning that subsequent waves generated between the initial time ''t''=0 and final time ''t'' will form similar triangles with coinciding right endpoints to the one shown. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cherenkov radiation」の詳細全文を読む スポンサード リンク
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