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Definition: the velocity with which phase fronts propagate in a medium
The phase velocity of light is the velocity with which phase fronts propagate in a medium. In vacuum, the phase velocity is c = 299'792'458 m/s, independent of the optical frequency, and equals the group velocity. In a medium, the phase velocity is typically smaller by a factor n the refractive index, which is frequency-dependent (→ chromatic dispersion). In the visible spectral region, typical transparent crystals and glasses have refractive indices between 1.4 and 2.8. Semiconductors normally have somewhat higher values.
In the X-ray region, refractive indices are slightly below 1, corresponding to phase velocities slightly above the vacuum velocity of light. Similar effects can be caused e.g. by optical resonances of atoms in the visible spectral region. However, this does not allow for superluminal transmission of information, or for violating causality.
The phase velocity is usually considered for plane waves, where the wavefronts are simply moving in a direction perpendicular to their orientation. It is interesting to look at the wavefronts of focused laser beams. Due to the Gouy phase shift, they move slightly faster near the focus.
See also: velocity of light, refractive index, group velocity, dispersion
Categories: general optics, physical foundations
