Is the speed of light a constant

[George Jones]

Let me cloud the picture further; the group velocity of a pulse of light in an anomalously dispersive medium can be greater than c.

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

 

[lightarrow]

Infact what count is "signal velocity" which however is not always very easy to define.

 

[George Jones]

Infact what count is "signal velocity" which however is not always very easy to define.

Yes.

In a medium with normal dispersion and n > 1, most of the energy propagates at a speed equal to the group velocity (less than c), but there is an initial small signal, called a precursor, that propagates almost at c. So, if one has sensitive detectors, signal velocity in any medium is very close to c. Sommerfeld was one of the first people to work this out, and I first learned about this in Jackson.