# Three Velocities of Light ?

1. Dec 31, 2008

### referframe

I believe there are three possible velocities of light: Phase, Group and Signal? Is that correct? Does a standing wave have a Phase velocity of zero? What is an example in which the Signal and Group velocities would be different? Obviously, I need clarification on this whole subject. Thanks in advance.

2. Dec 31, 2008

### clem

The P, G, and S velocities are defined for a traveling wave, and do not apply for a standing wave.
$$v_G=\frac{\partial\omega}{\partial k}$$ is the signal velocity if the higher terms in a Taylor expansion of $$\omega(k)$$ are negligible. That is when
$$\omega$$ is a slowly varying function of k.

3. Dec 31, 2008

### George Jones

Staff Emeritus
From Jackson (page 319 in the second edition):

"The general usage is to take the group velocity of the dominant frequency component as the signal velocity and velocity of transport. This suffices in most circumstances, but with sensitive enough detectors the signal velocity can evidently be pushed close to the velocity of light in vacuum, independent of the medium."