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This question already has an answer here:

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?

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marked as duplicate by Aaron Stevens, John Rennie, Chair, Jon Custer, ZeroTheHero Feb 10 at 21:46

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  • $\begingroup$ Have a look at How can a particle in a medium ever travel faster than light? and see if it answers your question. If not, prehaps you could edit your question to be more specific about what you are asking. $\endgroup$ – John Rennie Feb 10 at 6:08
  • $\begingroup$ @JohnRennie please upvote the question so that i can enter the chat rooms (I know these types of comments are accepted but what to do I want to get into h bhar):/ $\endgroup$ – user221098 Feb 10 at 7:28
  • $\begingroup$ It is the sonic boom equivalent (airplanes going faster thant he speed of sound in the air.) hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel.html $\endgroup$ – anna v Feb 10 at 14:40
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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 $3.00\times 10^8\,\rm m\,s^{-1}$.

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As Dicul Smerd explained the limitation on velocity of massive particle is the speed of light in free space. That is 3*10^8 m/s. Now you should not think of this restriction as the speed of light. It is actualy the speed of causality. If you have studied spacetime diagram you should know When some massive particle exced C it it violate the fundamental Cause --> Effect chain rule. A particle moving faster than C can travel back in time. Which is absurd according to relativity. Now Chrenkov radiation is similar to this event when a classical body moves faster than the speed of sound in that medium.

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Cherenkov's radiation (or effect) is radiation of the medium consisting of charged particles; in other words, the source of radiation is more complicated than just one charged particle. The resulting EMF may have properties different from the "vacuum" medium just because the source is spatially distributed charged particles with a specific affection by a fast projectile.

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