# Phase and group velocities

by Yoni
Tags: phase, velocities
 P: 455 "gw = c^2. The group velocity times the wave velocity is constant." That is true in a wave guide in vacuum. In a dispersive medium, where n depends on the frequency, $$v_{group}=\frac{dk}{d\omega}$$ and $$v_{phase}=\frac{\omega}{k}=c/n$$. Then their product is not a constant.