Cerenkov radiation is
the radiation produced by particles moving though a material at speeds greater than the
speed of light in that material.
In the Doppler Effect we can show the change in the
observed waveform due to a moving source (or observer). However if the velocity of the
source (v) is greater than that of the wave in the medium through which the wave is travelling (cm) we have a pheneomena called the Mach effect. This
problem was solved by Mach in 1936 (he received the Nobel Prize for Physics in
1958).
The envelope of the waves received by the observer will be a cone of semi
angle a, where sin a= cm/v. The
greater the value of v, the narrower the cone will be.
Examples of this are a sonic
boom, the bow wave from a ship and shock waves from a bullet.
The same effect is
also observed with light, as Cerenkov radiation. In some materials electrons from an
accelerator can move faster than the velocity of light in that material and as the electrons
travel through the solid or liquid a bluish radiation, a kind of shock wave, is observed. (for
example, in water-moderated nuclear reactors). This shock wave is called Cerenkov
radiation and the cone of waves emitted is electromagnetic in nature and usually in the blue
region of the spectrum probably because the speed of blue light in the material is less than
that of red light.
The diagram below should help you to understand what is
happening.
