On Thu, 27 Feb 2003, John Nichols wrote: > > I remember one of my physics lecturers at ANU saying that the change in > phase angle for light may travel faster than the speed of light. But then > again I could just be remembering it wrong from 25 years ago.
You remembered correctly, John. In a material medium: v(phase) times V (group) = c^2. As v(phase) becomes very large, even greater than c, v(group) becomes very small, but their product remains c^2. Only the "group" carries information, at v(group). In free space both v(phase) and v(group) are equal to c, and remain equal to c. In most material media, v(phase) depends on frequency so the components which build the signal group are dispersed and the envelope of the group becomes distorted as the signal group propagates through the medium. Gene.
