Roy,
I haven't reviewed the physics of relativity lately, but I 'believe'
that value of C is specified as the velocity of energy propagation in a
vacuum... SInce that limit is not violated when the wave front is
propagating in a medium with a refractive index greater than vacuum,
i.e. C is slower than in a vacuum, then by O'Connor's Law of
Reciprocity the limit will not be broken if the the wave front is
propagating a medium with a refractive index less than a vacuum, i.e. C
is greater than in a vacuum...

denny / k8do