Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Wave questions and Čerenkov radiation

  1. Sep 6, 2008 #1
    I have a couple questions about waves and Čerenkov radiation:

    1. I know in all acoustic waves, rarefaction occurs after a wave passes through a medium. On paper, is the trough of a wave its rarefaction? What is the force that causes rarefaction? I've heard the "atoms are like springs" explanation, but I don't quite understand it. I am trying to understand why the air particles in this picture rush towards the center. Why doesn't rarefaction occur in explosions (which have just one gigantic wave followed by much smaller ones) or in light waves? (or does it?)

    EDIT: Explanation of picture can be found here, but it doesn't explain why rarefaction occurs.

    2. In Čerenkov radiation, I understand that the phase velocity in a charged particle exceeds the speed of light (in that medium). I also understand that it is possible for lasers to have a higher group velocity than light. How can phase and group velocities (or even frequencies in some cases I believe) be faster than the signal (speed) velocity of a wave? Does the group/phase velocity dissipate when it gets to the end of a signal?

    Last edited: Sep 6, 2008
  2. jcsd
  3. Sep 8, 2008 #2
    Anybody? Should I post this in another part of a messageboard, or is my question not clear?

    Last edited: Sep 8, 2008
  4. Sep 8, 2008 #3

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    I admit an initial reluctance to respond. Your original questions are not very clear to me- for example, question #1 indicates some conceptual confusion between pressure (extrapolated to a discrete, particle model) and force, and question #2 seems like a 'word salad'. For example, I don't know what you mean by 'I also understand that it is possible for lasers to have a higher group velocity than light.'.

    Maybe framing the questions with a coherent framework would help: Is there a particular physical phenomena you are thinking about?
  5. Sep 8, 2008 #4
    In question #1 I was asking why rarefaction occurs, if it is represented on a graph by the wave trough, and what other types of waves besides sound waves (explosion, light, ocean etc.) encounter rarefaction.

    In question #2 I was asking how group or phase velocity can be faster than the wave signal. I was also asking what would happen if the group/phase velocity (or frequency) was fast enough to get the end of a signal because it was faster than the speed of the signal.
  6. Sep 9, 2008 #5
    rarefaction does occur in explosions. the momentum of the air carries it outward then the pressure drives it back.

    phase velocity can exceed c because nothing is actually moving at that speed. its a mathematical artifact. nothing more. group velocity on the other hand is usually descrided as the rate at which energy moves. why it can exceed c is beyond me.
  7. Sep 9, 2008 #6


    User Avatar
    Science Advisor

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Wave questions and Čerenkov radiation
  1. Waves and radiations (Replies: 1)