# What is Cherenkov radiation? [duplicate]

What is Cherenkov radiation? I came across this concept when I was learning SR. Why can't particles moving at speed of excess of speed of light in the medium. Why it does not violate the limitation is speed?

## marked as duplicate by Aaron Stevens, John Rennie, user191954, Jon Custer, ZeroTheHeroFeb 10 at 21:46

In certain phases of nuclear research and technology, a luminous glow may be observed where high- energy particles are moving through transparent matter. This is known as Cherenkov radiation, and is the result of particles moving through matter at speed in excess of the speed of light in that medium. This does not constitute a violation of limitation is the speed of physical bodies to values less than $$c$$ for in relativity the speed is light always and invariably means its speed in a vacuum. Since the index of refraction of water is about $$\frac{3}{4}$$, light travels at about $$2.25 \times10^8 \,\rm m\,s^{-1}$$ in water and Cherenkov radiation would be produced by particles having a velocity greater than this value. However, all such speeds would be less than $$c$$ which is not $$2.25\times 10^8 \,\rm m\,s^{-1}$$ but $$3.00\times 10^8\,\rm m\,s^{-1}$$.