There are other cases where strange dispersion effects cause velocities that
exceed c. The one I have seen most often is used in X-ray mirrors. The index
of refraction is less than 1, That means that the speed of light in the
material is greater than c. This results in total EXTERNAL reflection as
opposed to the total INTERNAL reflection we get in prism diagonals. When I
began studying X-ray astronomy this was quite a shock. A careful analysis
showed everything still made sense but I was never able to find an
explanation that wasn't really hard to understand.
The most interesting thing about this paper (to me at least) is that the
result is not in violation of any well established theory. The apparent
violations are the result of the oversimplifications that is an inescapable
consequence of physics without the math.
-Leonard