Cherenkov radiation

What happens

Nothing outruns light in vacuum, but light in water crawls at three quarters of its vacuum speed and a muon from the sky does not slow down to match. What follows is a cone at an angle that says exactly how fast the particle was going - cos θ = 1/(βn) - and that is why a tank of water with photomultipliers around it is a spectrometer. Below threshold there is NO light: not a faint cone, not a dim ring, nothing. Above it the cone opens fast, four degrees on the first per cent of energy, and settles toward the widest angle the medium can make - 41.39 degrees for water. The yield is about 340 photons a centimetre, which sounds like plenty until the ring arrives a metre across. On the bench the particle sheds photons as it crosses the radiator, each at the cone angle with an azimuth and a wavelength from Frank–Tamm, and they land on a detector plane: the ring BUILDS, blue because the spectrum is 1/λ², and as wide as the radiator because light born at the entrance has further to spread than light born at the exit.