Cherenkov Radiation
The optical sonic boom — and why nuclear reactor cores glow blue.
When a charged particle moves through a medium of refractive index $n$ with speed $v > c/n$ — faster than light in that medium — it emits coherent radiation in a cone of half-angle
$$\cos\theta_C = \frac{c}{n v} = \frac{1}{\beta n}.$$Threshold: $\beta n = 1$. For water ($n \approx 1.33$): $\beta_{\rm thresh} \approx 0.75$, so electrons with kinetic energy above ~ 264 keV radiate. The spectrum (Frank–Tamm formula) is broadband, with intensity rising linearly in $\omega$ until absorption cuts off — hence the characteristic blue glow in spent-fuel cooling ponds.
Cherenkov detectors (Super-Kamiokande, IceCube, ANTARES) measure cone angle to determine particle species + direction. Synchrotron radiation (charged particle on a curved trajectory) and transition radiation (crossing a dielectric interface) are related phenomena involving photons emitted by accelerated charges.
Cherenkov was awarded the 1958 Nobel with Frank & Tamm; the analog for matter waves in superfluid helium is dissipation above the Landau critical velocity.