## Main Question or Discussion Point

Hi all.
I don't understand how charged particles can travel faster than the speed of light in that substance.
I know SR still holds , and why does it produce a blue glow?
Thanks for any help.

Related Special and General Relativity News on Phys.org
ehj
Well.. the only thing that SR would state is that it's impossible to accelerate a partikle to c, the speed of light in vacuum. The speed of light in a medium is often lower than c though! So it is possible to send particles through a medium, that travel faster than a light wave in that medium, although not faster than c. The reason that it produces a blue glow is because accelerating charge emits radiation (I don't know why yet), why it is specifically blue is probably related to the energy of the particles.

Well.. the only thing that SR would state is that it's impossible to accelerate a partikle to c, the speed of light in vacuum. The speed of light in a medium is often lower than c though! So it is possible to send particles through a medium, that travel faster than a light wave in that medium, although not faster than c. The reason that it produces a blue glow is because accelerating charge emits radiation (I don't know why yet), why it is specifically blue is probably related to the energy of the particles.
Cerenkov radiation is similar to the sonic boom from a jet aircraft. Think of the aircraft as a particle exceeding the propagation speed of sound in the medium. The sound waves pile up and amplify, because the source of the sound waves is keeping up with the emitted wavefront, so that all the wave fronts are superimposed on each other. The radiation is more due to the charged particle disturbing electrons in the atoms of the medium as it passes through it, because the particle is not necessarily accelerating through the medium.

tiny-tim
Homework Helper
Hi kev!

But why blue?

ehj
"The radiation is more due to the charged particle disturbing electrons in the atoms of the medium as it passes through it"
And how do the disturbed electrons emit radiation then? The radiation is not caused by excitations in the atoms, but as you said yourself by the sonic boom effect, and the fact that accelerating charge gives off radiation. It is the acceleration that gives rise to the radiation. The charged particles are decelerating as they bump into other atoms but it's the deceleration of the charged particle that causes the radiation. Cerenkov radiation is a continuous spectrum and therefore not caused by excitations in the atoms it bumps into.

tiny-tim
Homework Helper
Cerenkov radiation is a continuous spectrum and therefore not caused by excitations in the atoms it bumps into.
Hi ehj!

But why blue?

ehj
well.. I think it has something to do with the energy of the charged particle, and how fast they decelerate. You can try reading http://en.wikipedia.org/wiki/Cherenkov_radiation
It says in there that the spectrum is continuous and most of the radiation is ultraviolet. It's therefore likely that what we're seeing is the part of the spectrum with low energy, the blue light, because right after blue light the ultraviolet comes.

well.. I think it has something to do with the energy of the charged particle, and how fast they decelerate. You can try reading http://en.wikipedia.org/wiki/Cherenkov_radiation
It says in there that the spectrum is continuous and most of the radiation is ultraviolet. It's therefore likely that what we're seeing is the part of the spectrum with low energy, the blue light, because right after blue light the ultraviolet comes.
The wikipedia article you linked to says

"As a charged particle travels, it disrupts the local electromagnetic field (EM) in its medium. Electrons in the atoms of the medium will be displaced and polarized by the passing EM field of a charged particle. Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed. (In a conductor, the EM disruption can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, when a disruption which travels faster than light is propagating through the medium, the photons constructively interfere and intensify the observed radiation."

I read that as the radiation being emitted by the electrons in the atoms of the medium disturbed by the passing charged particle.

tiny-tim
Homework Helper
Hi ehj!

Thanks for the wiki reference

Looking at it, it seems that all charged particles going through a medium create Cherenkov waves, even at low speeds (and that this depends on speed but not on acceleration);

but that these waves always destructively interfere, so causing no effect;

except if the particle is moving faster-than-light-in-the-medium, and then only in the "electromagnetic boom" shock-wave caused by the wave fronts crashing into each other, in which they constructively interfere, producing actual photons.

I think.
It says in there that the spectrum is continuous and most of the radiation is ultraviolet.
But why is most of it ultraviolet?

Does it depend on the temperature of the medium? Or its density? Or some gradient feature?
Maam! …
But why blue?

if thats true then Cherenkov radiation should be produced whenever radiation enters or leaves a substance regardless of the speed of the particles

tiny-tim