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Erenkov radiation: Why is it possible?

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Hunterbender
#1
Jan11-09, 12:03 AM
P: 54
Well, I was reading about neutrino the other day, and I came across the Čerenkov radiation. I understand that the universal speed limit is the speed of light, c, in a vacuum. I understand that Čerenkov radiation occurs when objects (not as physical object, but stuff) travel faster than light in an alternate medium. What I don't understand is:

How is it that a medium exists that can slow down light more than some other substance? For example, air: it slows down a person and the bike the person is riding at the same time. How is it that this substance exists that can slow down one thing (ie light) more than other things (also in the same medium)?

Also, is there special characteristic of these substances?


(Note: I am still a high school senior taking basic differential equations at school, please refrain from writing fancy 2 page long equations/formulas. Thanks!)
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vanesch
#2
Jan11-09, 12:11 AM
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Sci Advisor
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P: 6,236
Quote Quote by Hunterbender View Post
Well, I was reading about neutrino the other day, and I came across the Čerenkov radiation. I understand that the universal speed limit is the speed of light, c, in a vacuum. I understand that Čerenkov radiation occurs when objects (not as physical object, but stuff) travel faster than light in an alternate medium. What I don't understand is:

How is it that a medium exists that can slow down light more than some other substance? For example, air: it slows down a person and the bike the person is riding at the same time. How is it that this substance exists that can slow down one thing (ie light) more than other things (also in the same medium)?
What happens is that in many media, the electromagnetic wave which is light sets into oscillation charges (the electron clouds of the composing atoms and so). These oscillating charges on their turn send out electromagnetic radiation which is perfectly coherent with the incident radiation, but which has a phase shift. The combination (interference) of the incoming radiation and this emitted radiation make that there is an overall shifting of the phase in the material which is proportional to the amount of material traversed: that's nothing else but an effective slowing-down of the original wave. So you can consider this as two different situations:
You can consider that light always travels at c (as in a vacuum), but take into account all these secondary sources (which also send out light at c).
Or, you can only consider the the initial light, without the extra sources, but with a modified velocity. The modified velocity is a "trick" which allows you to take effectively into account all the wave-atom interactions.

Now, if a wavesource travels faster than the effective wave it emits, you get constructive interference of all wavefronts on a conical surface. With sound, that's the sonic boom, and with light, that's Cherenkov radiation.
Hunterbender
#3
Jan11-09, 01:04 AM
P: 54
First of all, thank you so much for responding.

I am wondering if I am understanding this correctly. So there are two different measurement of "speed" (or velocity I guess). One of which measure the velocity of its oscillating charges (which has some phase). The other is the velocity of the wave (of the light).

In a medium, the phase of the oscillating charge changes more than the velocity of the wave, so it "break" through in a phenomenon we call Cerenkov Radiation. Is that correct?

Then, (sorry a few more questions pop up, if you don't mind) does all medium demonstrate such an effect (separating the wave into oscillating charges and wave itself)?

Thank you for your time.


Ben Zheng


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