A dielectric material does slow down the rate at which information is
transmitted by light, which is given by its 'group velocity'. Usually
light's 'phase velocity', which determines the refraction of a
continuous plane wave when it encouters regions of different refractive
index, is also reduced below the vacuum value, c, but not always. The
phase velocity can be bigger than c. /mike w.
"Information" can still travel at speeds up to the speed of light,
however. In particular, high-energy particles can be thrown at
dielectrics, and before they slow down, they are still traveling at
close to the speed of light (in vacuum, not in the material).
Cosmic-ray muons are constantly flying through materials, dielectric or
no, it makes no difference, and it takes a lot of material to slow them
down. In the mean time, because they are charged, they carry with them
the electromagnetic field of a moving, charged object. Since the muons
are moving faster than the phase velocity of light in a material, they
create a "shock wave" of Cherenkov radiation as they traverse a
dielectric. Some physicists use this radiation, which gets emitted in a
cone around the muon flight path, to detect muons in big tanks of water
or oil.
Tom
(published on 10/22/2007)
