Why is Cherenkov Radiation faint blue in colour?

Question

In 1934, Cherenkov discovered that electrons moving with large constant velocities through polarizable media caused a faint bluish glow. 'Why is the radiation blue in colour? How can a charged body moving with constant velocity emit an electromagnetic radiation?'

Answer 1

'Why is the radiation blue in colour?

From the wiki article

The frequency spectrum of Cherenkov radiation by a particle is given by the Frank–Tamm formula. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.

Italics mine.

How can a charged body moving with constant velocity emit an electromagnetic radiation?'

It moves with constant velocity until it meets and interacts with the field of an atom/molecule.

From the wiki link again:

As a charged particle travels, it disrupts the local electromagnetic field in its medium. In particular, the medium becomes electrically polarized by the particle's electric field. If the particle travels slowly then the disturbance elastically relaxes back to mechanical equilibrium as the particle passes. When the particle is traveling fast enough, however, the limited response speed of the medium means that a disturbance is left in the wake of the particle, and the energy contained in this disturbance radiates as a coherent shockwave.

Answer 2

When high velocity particles travel faster than the speed of light in a medium they create a blue flash. It should be noted that only electrically charged particles are capable of emitting Cerenkov radiation. The analogy we can use is that it is the speed of light equivalent to the speed of sound sonic boom.

As a supersonic jet accelerates, the air piles up in front of the leading edges of the wing and tail of the aircraft. The sonic boom is caused by a sudden pressure drop as the aircraft moves faster than the air molecules can get out of the way.

The speed of sound is determined by how fast the air molecules can move. 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 (and this pressure drop creates the distinctive double sonic boom). The pressure waves subsequently moves away from the wing at the speed of sound. 

Image Source: Argonne National Laboratory - originally posted to Flickr as Advanced Test Reactor core, Idaho National Laboratory

Turning to Cerenkov radiation, the electric field carried by a charged particle moves along at the same speed as the particle. The electrical field is limited by the fact that it can only travel at the speed of light. If the particle is travelling faster than the speed of light in a given medium (such as water), then it sometimes can "get ahead" of the electric field it carries.

So now we have the electromagnetic version of the sonic shock wavefront. This shock wave is made of blue light, for two basic reasons.

The first is that in the water which is used as a moderator in some nuclear power plants, it is a characteristic blue color because of the specific atomic structure of water. The shock waves causes a raising of the energy levels of electrons of water, which absorb the light shock wave and then re-emit it, at the blue end of the electromagnetic spectrum, as you can see in the picture above.

The second 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 shifting the spectrum to the blue side.

So why the link between nuclear reactors and the distinctive blue light?

Light propagation speed in water is reduced to 3/4 of it's value in vacuum. There are lots of fast moving, (actually very fast) moving particles emitted by the nuclear fuel and it's decay products. In the water surrounding the fuel rods, the speed of light drops to 0.75c. and thus the neutron speed at which Cerenkov radiation can no longer happen. Matter particles, usually electrons, can then "outrun" the electric charge they carry, through an electrically polarizable medium, such as water.

In water cooled nuclear reactors, beta particles are emitted but the blue glow can continue to be seen after the heat producing moderated chain reaction stops and the fission products decay. Cerenkov radiation also acts as a marker for the residual radioactivity of nuclear fuel rods due for burial or reprocessing.

Answer 3

If you understand galactic redshifting then this is basically way on the other side of the spectrum in terms of scale, mechanics, and the literal spectrum of visible light.

The radiated particles are moving towards you faster than the light waves being emitted from atoms excited(energized) by the original radiated particles. This is possible because the speed of light is affected by different mediums(materials). The excited atoms emit light energy as their energy state returns to ground(normal) and the result is light waves hitting your eyes at a higher frequency(shorter wavelengths) than what they originally were when emitted from the affected particle(s), "blueshifting" the photons. This is known as the Doppler effect. The difference in Cherenkov radiation is that the particles are electrostatically charged so the radiated particle passes energy along rather than emitting light energy itself, and moving through a dielectric(non conductive) medium. Distilled water has extremely low conductivity and pure water has zero conductivity.

EXAMPLE

My analogy is imagine you are observing a car with no lights, and as that car drives down the road towards you, streetlights activate alongside it. In my analogy there is some point that the car can drive fast enough so it is unable to experience the light from the lamp it just activated(excited). So now the car is driving past and activating lamps faster than the speed of light in that given medium. This results in the car activating lamp B before the light from lamp A can reach lamp B. The same happens for lamps B and C, C and D, and so on, thus shortening the distance between waves and increasing the frequency of the emitted light for the observer.

(I understand this is quite a simplification. My answer is geared for less grammatically gifted people who could barely understand previous answers like myself.)