Speed of information in a medium

[Vanadium 50]

There is no such thing as "the initial EM wavefront"

True, in the sense one can never set foot in the same stream twice.

One can always split the material EM wave solution into two: the new solution is the sum of the orignial wave and the "response". There's no physics here, just subtraction.

So the real question is whether there is utility in thinking of things this way. Well, "sometimes" and "it depends". I can think of only one example (transition radiation) so its probably not helpful all that often.

 

[wnvl2]

As has been shown by Sommerfeld in 1907 by using an elegant analytical argument (theorem of residues) one can show that for arbitrary waves with compact spatial support the boundary of the support moves with the speed of light in vacuum inside the medium. That's understandable, because the medium can only be disturbed by and react to the incoming wave when this wave reaches it. Only then the medium emits its own electromagnetic waves which superimposes with the incoming wave.

Are you refering here to precursors?

https://en.wikipedia.org/wiki/Precursor_(physics)

Do you perhaps have a reference where it is explained as simple as possible?

So this implies that if you send an EM pulse wave through water a first very little signal arrives with the speed of light in vacuum at the other side of the water.

 

[tech99]

Are you refering here to precursors?

https://en.wikipedia.org/wiki/Precursor_(physics)

Do you perhaps have a reference where it is explained as simple as possible?

So this implies that if you send an EM pulse wave through water a first very little signal arrives with the speed of light in vacuum at the other side of the water.

I don't think this can be correct or else we would see a leading echo in optical fibre communication, which we don't.

 

[wnvl2]

But if I understand it correctly it will be very small as Dale said in post #26 because of the extinction theorem. That is probably why we do not detect it in practice.