The use of a periodic diffraction grating can produce this effect at any velocity, and photonic crystals can force this effect to occur at obtuse angles, its reverse-cone form.
States)  The Whipple observatory originated the use of this effect to detect atmospheric high-energy gamma ray photons. The use of a periodic diffraction grating can produce this effect at any velocity, and photonic crystals can force this effect to occur at obtuse angles, its reverse-cone form. This effect results from an excess of negative charge in secondary particle showers, scaling with the square of net charge, according to the Askaryan effect, while the former type of this effect is known as the Smith-Purcell effect. The energy emitted per unit length by this effect is proportional to the integral of permeability times frequency times quantity one minus one over beta squared refractive index squared; that integral is performed over all frequencies such that velocity is greater than speed of light over the refractive index, the Frank-Tamm formula. FTP, name this cause of a nuclear reactor's blue glow, the consequence of charged particles traveling faster than the speed of light in a medium.
States)  The Whipple observatory originated the use of this effect to detect atmospheric high-energy gamma ray photons.
{Cherenkov} radiation