Cherenkov radiation

The speed of light varies depending on the medium it is traveling in. Cherenkov radiation is emitted when particles move faster than the maximum light speed through a non-vacuum, leaving a wake of light trailing behind. This radiation helps determine the speed of ultrafast particles.
 Pavel Alekseyevich Cherenkov Russia 1934 Cherenkov radiation

The speed of light varies depending on the medium it is traveling in. Cherenkov radiation is emitted when particles move faster than the maximum light speed through a non-vacuum, leaving a wake of light trailing behind. This radiation helps determine the speed of ultrafast particles.