A sources of waves which is stationary will emit waves uniformly in all directions.
The waves move at a speed defined by the equationIf the source itself is moving at a speed close tothen it nearly keeps pace with its own spherical wave fronts as shown in the top diagram below.
In the second diagram the speed of the source is larger than the speed of the wave. When the source S was at positionit generated wavefrontand when it was at positionit generated wavefrontAll the spherical wavefronts expand at the speedand their boundaries move along the surface of a cone. In the case of a wave source in a fluid like water or air, the cone signifies a shock wave and is referred to as the Mach cone. The surface of this cone has half-angleand is tangent to all the wave fronts. A similar effect occurs for light when an electrically charged particle traverses a dielectric medium at speeds that are greater than those of light in that medium. In this case, the electric and magnetic fields associated with a rapidly moving charge excite the atoms of the medium. The excited atoms emit part of their light in the form of a coherent wavefront of radiation at fixed angle with respect to the trajectory of the charged particle as shown below.
To be precise, this radiation, named Cerenkov radiation after its discoverer, is produced whenever the velocity \beta c of the particle exceedswhereis the speed of light in a vacuum,is the refractive index of the medium traversed by the charge, andis the usual relativistic factor
ifis the velocity of the particle. For most cases, energies are high enough for us to assumeFrom the diagram above, we can see that the light cone formed in Cerenkov radiation has a valueThe radiation appears as a continuous spectrum. In a dispersive medium, bothandare functions of the frequency of radiation. The number of photons at a particular frequency or wavelength, as it turns out, is proportional to This means that, in the visible range, blue light predominates over all other colours. The blue glow that emanates from the water in which highly radioactive nuclear reactor fuel rods are stored is caused by the Cerenkov effect. For fuel rods, much of the radiation they emit is in the form of high energy electrons. The electrons travel through the water at a velocity greater than that of light in water and hence cause the characteristic ``Cerenkov glow''.