For a particle travelling faster than light, the wave-fronts do overlap, and constructive interference is possible, leading to a significant, observable signal. Click here to view animations of the radiation from a moving particle. A particle can not, of course, travel faster than the speed of light in a vacuum. As can be seen from the above diagram, a cone of light radiates out from each point on the particle's track. It can be shown that the number of photons per unit length of track is given by n is a function of the frequency , for most materials decreasing rapidly in the ultraviolet.
Most radiation is therefore in the visible, peaking at the blue end of the spectrum.
In the visible range, the number of photons is roughly 500 sin 2 C per centimetre of track. Particles with a velocity above the threshold for producing Cherenkov light are detected, while others are not. If a gas is used as the radiator, its refractive index, and so the threshold velocity, can be tuned by adjusting the pressure. Differential Cherenkov detectors are slightly more sophisticated, in that they respond to a range of velocities. In this way, the centre of the ring indicates the position of the particle and the radius of the ring measures the Cherenkov angle and so the velocity of the particle.