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Q & A: dielectric material slowing light

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Most recent answer: 10/22/2007
Q:
Does a dielectric material slows down the ultimate speed at which information can travel or it just slows down the phase velocity of light.
- Azeem (age 25)
Pakistan
A:
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)

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