"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

"Josef Matz" wrote in message
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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

I was in a hurry this morning and didn't ask my main question.
I think at this point I understand different frequencies travel at different
speeds.
Group Velocity vs Velocity Factor what is the difference?
If vg = c * sqrt(1 - (f/fc)^2) hmm, maybe I should tell what I think I know.
(I'm way over my head on this subject).
If I generate a spark ( many frequencies) all these frequencies combine to
make a waveshape, as the wave travels down the waveguide the waveshape
changes because different frequencies are traveling at different speeds?
Correct me as needed.
So is 'group velocity' the velocity the peak of the signal as it travels
down the waveguide?
Forgive my ignorance but the formula vg = c * sqrt(1 - (f/fc)^2) doesn't
work for me. (1-(f/fc)^2) is negative and I can't get the sqrt of a
negative. What did I miss?
Thanks, Mike