(radiation from the shock of a charged particle passing through a medium)
Cherenkov radiation is electromagnetic radiation (EMR) emitted when a charged particle,
e.g., electron, passes through a dielectric medium at a speed
faster than the speed of light within that medium.
The electromagnetic disturbance at the point where the particle
travels moves faster than light and causes EMR in
an expanding cone shape (Cherenkov cone),
with an emphasis on shorter wavelengths.
A well-known example is the blue glow in pictures of cooling water
pools within nuclear-reactor facilities.
Spaceborne gamma rays can trigger showers of particles
(such as electron-positron pairs) traveling at high speeds
in the atmosphere, in turn, generating Cherenkov radiation in
the air that can be detected by Earth-bound detectors, enabling
them to indirectly detect the gamma rays (Cherenkov detectors).