On 25 Oct 2003 14:01:38 -0700, (EL) wrote:

Randy Poe wrote in message . ..
[EL]
Hi Bilge.

Why is it so difficult for people to imagine group velocities as wave
envelopes!

All decent empirical wave-group-velocity measurements show that they
are much lower than phase velocity in the same dispersive medium.

Except in negatively-dispersive media, where both theory and
experiment show the opposite.

[EL]
Bring back the grudge and let us grind.
What is the merit in being a beautifully coloured parrot?
The merit is to admire the colourful feathers (mostly in a mirror).
However, a parrot is a sound mimicking bird with a bird's brain.

Nice poetry, if unfathomable. Shall we discuss physics?


Dispersion is another word for scattering.

Incorrect.

Dispersion in wave physics means wavelength-dependence of the speed of
propagation. A medium can be dispersive but, in theory, lossless.
Scattering is inherently lossy. It refers to energy being sent in many
other directions away from the line of propagation.

Normal (positive) dispersion does have the tendency to spread signals
out in time, but this is a different phenomenon than dispersion.

Scattering if you did not know is like what happened to the Jews all
over history.
Scattering is like holding a handful of seeds and then tossing them
for random distribution during planting processes.

Again, nice poetry, but not very closely related to either dispersion
or scattering as the terms are used in physics.

A dispersive medium is a medium as media are defined in being either
homogenous or not and isotropic or not.

A dispersive medium is one in which speed depends on frequency. As we
all know from prisms, glass is a dispersive medium. Most real media
area, though the slope depends on the details of the interaction
between the energy and the molecules of the medium. Most of the time,
the slope of a speed vs. wavelength curve is positive, i.e., the speed
increases with wavelength, or decreases with frequency. Low frequency
waves propagate faster. You can see this in water with storm systems,
with low frequency components traveling reaching shore far ahead of
the rest of the system.

But there are materials which exhibit an opposite slope over some
wavelength regimes, with higher frequencies having higher speeds. This
is not magic, but it leads to a different effect on the shape of a
multiple-frequency pulse as it propagates.

What does the idiotic negative-dispersion supposed to mean!

Indicated above.


Pick up any respectable reference that tabulates the refractive index
of materials and try to find any negative value.
The overwhelming majority of indexes have a value between 1 and 2 and
they are POSITIVE VALUES.

Dispersion refers to slope. How low frequencies move compared to high
frequencies is what leads to the effect.


Now the rate of change of positive values over positive intervals is
quite unlikely to make sense being negative.

Huh? You're saying it doesn't make sense for a positive value to
decrease? Why the hell not?

http://en.wikipedia.org/wiki/Dispersion_(optics)
http://www.corning.com/opticalfiber/...ture_page2.asp
http://adsabs.harvard.edu/cgi-bin/np...ptCo.179..107W

- Randy