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 http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/monopolfinalptpt.gif" [Broken] 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 http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-basics-pointsources.htm" [Broken], 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? Thanks!