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Speed Of Light In Materials

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Posted by yanniru/">yanniru on July 6, 1999 12:53:51 UTC

The statement in this forum has been made that the speed of light never varies and I was challenged to prove otherwise and provide references. My reference is Born & Wolf, Principles of Optics. In there you will find that when light propagates in materials, its group velocity slows down. Ther are so-called precursors that propagate through the materials at the speed of light, but precursors can carry no information. For example, lets consider the simple approximation for the dielectric constant of a plasma. In MKS its E/E0 = sqrt(1-omega**2/omega0**2) where omega is the radial frequency of light. This formula yields the group velocity in the plasma, the speed at which information travels: V/C = sqrt(1 - omega**2/omega0**). By the way, omega0 is the so-called plasma frequency. We can see from this formula that the group velocity is a function of frequency. In fact, when the light frequency equals the plasma frequency, the group velocity equals zero. This is when the plasma becomes opaque. I might mention that when the group velocity slows down, the phase velocity of light speeds up, so that when light propagates through materials, its phase velocity is faster than the speed of light in a vacuum. So the speed of light and its group velocity and phase velocity are constant in a vacuum, and the entire spectrum travels at that speed. But in materials, all these velocities change as a function of frequency. Its commonly called dispersion.

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