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When charged particle inside a media moves faster than the speed of light of that media, a faint bluish glow is observed which we call as "Cherenkov radiation". It is a very common scenario in nuclear power plant. I have been told that the high energy particles polarise the dielectric media (water) while traversing through them. When they depolarize they emit photon. The energy difference between excited and ground state of media particles is such that the frequency of emitted photon falls in blue light region of visible spectrum. That's why we see blue colour.

My question is, if it is so, then what is the necessity of charged particles moving faster than light? I mean, can't they polarise the water molecule even if they move slower than speed of light?

Moreover, in wikipedia, I have read that it is also possible to observe Cherenkov radiation in vacuum. So it seems there is no connection between Cherenkov radiation with medium and speeding faster than light. Is it so?

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    $\begingroup$ Have you read the Wikipedia article on Cherenkov radiation? If so was there something that didn't make sense in it or needed more explanation? $\endgroup$
    – Triatticus
    Commented Aug 6, 2020 at 14:22
  • $\begingroup$ Yes. I am totally confused about Cherenkov radiation source. $\endgroup$
    – Sabbir Ahmed
    Commented Aug 6, 2020 at 14:40

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Cherenkov radiation isn't actually a new mechanism for producing light. Rather, it causes the light that is already being produced to bunch together into a bright cone. The effect is exactly the same as a sonic boom, only in this case, we observe a cone of bright light. There are many mechanisms by which a charged particle can emit radiation when moving through a medium. Even in classical mechanics, if the particle is slowing down as it passes through the medium, it will emit radiation, because accelerated charges radiate. The radiation emitted by the particle due to this effect may be very small. However, when the particle is travelling faster than the speed of light in the medium, any radiation being emitted by the particle will bunch up into a cone, making it clearly visible to detectors.

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