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? Thanks!