Past exam of the optics and electrodynamics course of the University of Cambridge / 2016 / Paper 1 / 1 / a / Solution

Interpreting the cone as Cherenkov radiation, its angle gives the speed, not directly the mass:Combining this with the relativistic kinetic energy $T=(\gamma -1)m{c}^2$ gives the Cherenkov mass inferenceThe material must be transparent at the observed frequency, $n$ is its phase refractive index there, and $n\cos \theta >1$. For $T>0$ this requires $0<\theta <\arccos (1/n)$ for a massive particle above threshold. Radiation of an unspecified other origin would not determine the mass from these data alone.