jawod wrote:
. . .
I do not understand how EMR can travel in any medium "faster" than in a
vacuum. Regarding Ray's post, I don't have a precise definition of
velocity factor but I suspect it is not describing wave propagation
directly. (And I am speaking as a total mathematical pedestrian
. . .

Suppose you put a 1 GHz signal into a waveguide and stopped time so you
could look at it. Measure the distance taken up by one complete cycle,
and you'll find that it's greater than c/f. (In a vacuum, the distance
is c/f, and in a medium with velocity factor less than 1 it's less.) Now
start time up again, and look at just one spot along the waveguide. Sure
enough, a whole cycle goes by every ns. Sure looks like it's going
faster than c. This is the phase velocity.

But if you turn the signal off at one end of the waveguide and measure
how long it takes for it to disappear at the other, dang. The change
travels at the group velocity, which is never faster than the speed of
light.

Roy Lewallen, W7EL