Cerenkov radiation The speed of light in a material medium, v, is less than the speed of light in free space c. The ratio of the two speeds, v/c is the refractive index n of the medium. According to the special theory of relativity, no particle can be accelerated to a velocity equal to c. But it is possible that a particle may move with speeds higher than the speed of light in a material medium. When an electrically charged particle moves through a medium at a speed exceeding that of light in that medium, a cone of light wave is emitted. This light which is visible as bluish glow is known as Cerenkov radiation.
The production of Cerenkov radiation is shown in the fig. c7. From the figure, the cosine of the angle between the direction of propagation of the radiation and the particle direction is,
cos q = [(c/n) t ]/ vpart t = c/(n vpart)
It is possible to determine the speed of charged particles moving in material medium, by measuring the angle q .