The group velocity is one version of the speed of light. When a short pulse of light moves through a medium other than vacuum, it can be shown experimentally that the individual crests of the light waves move at a different velocity to the pulse of light as a whole. The velocity of the individual crests is called the [Phase velocity]?. The velocity of the whole pulse is called the Group velocity.
The group velocity is in most cases a very good approximation to the speed that information can be sent through the medium using light.
In certain rare cases, such as Xrays through most solids, and also microwaves through a wave guide, the Phase velocity can be greater than the speed of light in a vacuum, but the Group velocity remains less that the speed of light in a vacuum.
Recently, some bizarre cases have been found where the group velocity is faster than the speed of light in a vacuum. It appears (though this may still be slightly controversial) that in these cases information still travels slower than the speed of light in a vacuum. (If information could travel faster than light in a vacuum there would be interesting consequences: see Theory of relativity.)
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