Can somebody explain phase and group velocity please. Specifically in reference to EM waves

Question

anonymous · Answer

Hmmm.. i think.. phase velocity is simple the velocity with which the wave is moving.. you see the wave to be moving with certain speed right??.. so that speed is basically called the phase velocity

However group velocity is when lot a waves are stacked up together.. (superposed ).. they give a resulting superposed wave right?.. and now that wave might look very different.. so at what speed that wave packet travels.. is group velocity!

anonymous · Answer

http://en.wikipedia.org/wiki/Group_velocity see the animation.. i know what your doubt is.. if different waves are travelling with different speeds.. how can the group as whole have one speed.. right? :D.. well the animation makes it clear

anonymous · Answer

So I can think of it as the rate the amplitude changes or the peaks move and the rate the wave travels?

radar · Answer

Phase velocity may exceed the speed of light.