The Cherenkov effect occurs if an electrically charged particle in a medium travels faster (with velocity v=βc) than the speed of light in this medium (with velocity c’=c/n, n being the refraction index). In this case, the particle emits light in form of a cone. The opening angle of the cone depends on the particle velocity and on the medium’s refraction index n: cos(θ) = c’/n=1/nβ. Experimentally, little light flashes produced this way can be detected for example via photomultipliers and hence turned into electrical signals. Using several photomultipliers, one could measure the cone opening angle and hence determine the particle’s velocity. The Cherenkov effect is the optical-electromagnetic analogon to supersonic planes as described in the movied on the right-hand side. Cherenkov Effect