Čerenkov radiation is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Čerenkov radiation.
Excerpted: Wikipedia
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Explanation through example: Electrically charged particles have electric fields around them as a result of their charge. When such a charged particle is moving, the electrical field moves along with the particle. However, since the electrical field is carried by photons, it can only travel at the speed of light. If the particle is travelling faster than the speed of light in a certain medium (such as water), then it, in a sense, out-runs its electrical field. This electrical field that is left behind forms a shock front, much like with sound. But, this shock front manifests itself in the form of light, not sound. As to why this light is blue, there are basically two reasons. In water, the blue light comes from excited atoms that emit blue light. The atoms in the water become excited by the Cerenkov shock wave and then de-excite, emitting blue light. But, another reason is that the number of photons emitted by such a charged particle is inversely proportional to wavelength. This means that more photons are emitted with shorter wavelengths, thereby tilting the spectrum to the blue side.
Excerpted: Physlink
Explanation through example: Electrically charged particles have electric fields around them as a result of their charge. When such a charged particle is moving, the electrical field moves along with the particle. However, since the electrical field is carried by photons, it can only travel at the speed of light. If the particle is travelling faster than the speed of light in a certain medium (such as water), then it, in a sense, out-runs its electrical field. This electrical field that is left behind forms a shock front, much like with sound. But, this shock front manifests itself in the form of light, not sound. As to why this light is blue, there are basically two reasons. In water, the blue light comes from excited atoms that emit blue light. The atoms in the water become excited by the Cerenkov shock wave and then de-excite, emitting blue light. But, another reason is that the number of photons emitted by such a charged particle is inversely proportional to wavelength. This means that more photons are emitted with shorter wavelengths, thereby tilting the spectrum to the blue side.
Excerpted: Physlink
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