Question

expression for group velocity ?
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Answer 1

Since each amplitude envelope contains a group of internal waves, this speed is usually called the group velocity, vg. In a given medium, the frequency is some function ω(k) of the wave number, so in general, the phase velocity vp = ω/k and the group velocity vg = dω/dk depend on the frequency and on the medium.

Answer 2

Answer:

It has small wavenumber and frequency and so a long wavelength and period. It travels with velocity vg = ∆ω/∆k. Often vp is known from measurements or from basic principles. Take the expression for vp and write ω/k for vp in it.