Group velocity wants to describe a pulse containing more than one photon
frequenccy.

In dispersive media the group velocity is a function of frequency of the
photons that form
a physical signal. So neighboured frequencies have a little different
velocities. That is what is behind.

So group velocity as one uses the terminus in hard physical theory is
nothing else as the true physical velocity at a certain frequency of the
photonic carrier resp. field.

Group velocities of wave packets is something apart from that. If you have a
carrier that containes
a spectrum of frequencies it describes the broadening of the signal due to
different carrier frequencies in
which have different velocities.
This definition is therefore unsharp and has only qualitative picturesque
meaning !

So group velocity in a sharp sense is just the real velocity which with the
field and the photons in move
at and only at a certain frequency.

Josef Matz


"amdx" schrieb im Newsbeitrag
...
Can someone explain how these two relate in a waveguide.
My limited understanding is, group velocity is slow near cutoff and
increases as frequency increases to almost c.
I don't know the difference between group velocity and phase velocity.
Thanks, Mike