### Question

When high velocity particles travel faster than the speed of light in a medium they create a blue flash. This is called Cerenkov Radiation, why does this happen and why is it blue?

Historically, Cerenkov radiation was discovered by Pavel Cerenkov in 1934, while he was studying the effects of radioactive substances on liquids. He noticed that water surrounding certain radioactive substances emitted a faint blue glow, which is now termed Cerenkov radiation.

Cerenkov radiation in the core of the nuclear reactor
Submitted by Charles Bell (who works at the plant.)

This radiation, as you pointed out, comes from particles travelling at a speed greater than the speed of light in the medium in which they are moving. The explanation we have adopted for Cerenkov radiation was first given by Tamm and Franc. To clarify your question a little, only electrically charged particles emit Cerenkov radiation. The reason they emit Cerenkov radiation can be explained using the more intuitive example of the sonic boom. Normally, when an airplane travels through the air, the wings push the air in front of it out of the way. However, the signal for air to move out of the wing's way can only travel at the speed of sound (in air). So if the airplane is travelling faster than the speed of sound, the air cannot move out of the way. This creates a sudden, intense pressure drop that moves away from the wing at the speed of sound, just like the wake behind a boat. It is the sound created by the pressure change that we hear after the airplane has passed over our heads.

Cerenkov radiation in the core of the McMaster University researcah nuclear reactor
Submitted by Anton Skorucak, creator of PhysLink.com (who used to work there.)

With this example in mind, let's get back to electrically charged particles... 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 our earlier example 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.
Answered by: Andreas Birkedal-Hansen, M.A., Physics Grad Student, UC Berkeley

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