Consider an electron approaching a sample of glass with a velocity close to the speed of light, c. As the electron moves through the glass light will instantly be emitted along its track, if its velocity is high enough. Even more, the electron will leave the glass sample before the light since the velocity of the particle inside the sample is larger than the speed of the light. At first sight, this seems to be in contradiction to Einstein's theory of special relativity, which states that nothing can travel faster than the speed of light, but one often forgets the important condition: in vacuum. This is the story of light emitted at particle travelling faster than the speed of light.
Top2. Cherenkov Light
During the early years of the 1930’s a Russian physicist, Sergei Ivanovich Vavilov, suggested to one of his students, Pavel Alexejevitj Cherenkov, to study the luminescence that occurred when gamma rays interact with different substances dissolved in water (Ekspong, 1958; Lennander, 1958).
Already in 1910 Marie Curie had found that bottles of concentrated radium- solution glow with an uncanny pale blue light. The first deliberate attempt to study the phenomenon was made by Mallet in 1929 (Mallet, 1929). He found that the light emitted from a wide variety of transparent bodies placed close to a radioactive source always had the same bluish white quality, and that the spectrum is continuous, not possessing the line or band structure characteristic of fluorescence (Jelley, 1958). He was the first to appreciate the generality of the effect and to notice that in a number of other aspects also, it is very different from fluorescence and other known forms of luminescence. Unfortunately Mallet did not pursue the work, nor did he attempt to offer an explanation for the origin of the light. The subject then lay dormant until Cherenkov began to study the phenomenon.
The delay in the study of the phenomenon was due to a number of causes. For instance, at the time of these early observations, much work was going on in the systematic study of the fluorescence and phosphorescence of materials irradiated by ultra-violet light, X-rays and the newly discovered radiation from the radioactive elements. The diverse and relatively complicated phenomena associated with these forms of luminescence only helped to postpone the discovery of Cherenkov radiation; the latter in any case was so weak that it was frequently masked by the presence of these other effects. The absence of really sensitive light detectors also contributed to the delay in the discovery of Cherenkov radiation; the early work had been carried out either by visual observation or by photographic recording with long exposure. Tamm has pointed out that the effect could have been predicted theoretically much earlier if a non-critical, oversimplified view of the implication of the theory of special relativity had not restrained thinking (Tamm, 1959). One often says that particles cannot travel faster than the speed of light and forgets the important addition: in vacuum. The maximum velocity for particles is the same(c), but the phase velocity of light in matter is lower (c/n), where n is the refractive index of the material.
Cherenkov started by examining the pure solvents of the substances. In 1934, his redistilled samples of water showed small luminescence when they were irradiated with gamma rays. Since water is not expected to fluoresce, this was a most remarkable result. Tamm has said that other physicists who were told about Cherenkov’s results made jokes about the hallucinations of a young man who had spent too long in the dark, or if they believed in the results, explained them as normal fluorescence (Tamm, 1958). Vavilov, on the other hand, made the hypothesis that the effect does not originate from the gamma rays but is caused by Compton electrons (Vavilov, 1934). This hypothesis was later confirmed experimentally by Cherenkov, who also showed that the effect could be caused by fast electrons from a β source.