RP Photonics logo
RP Photonics
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the

Phase Velocity

<<<  |  >>>

Definition: the velocity with which phase fronts propagate in a medium

German: Phasengeschwindigkeit

Categories: general optics, physical foundations

Formula symbol: vph

Units: m/s

How to cite the article; suggest additional literature

The phase velocity of light is the velocity with which phase fronts propagate in a medium. It is related to the wavenumber and the (angular) optical frequency:

phase velocity

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, called 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 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.

There are even cases where the phase velocity is directed opposite to the direction of the energy flow. Such phenomena occur in negative-index media, which can be realized as photonic metamaterials.

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
and other articles in the categories general optics, physical foundations

If you like this article, share it with your friends and colleagues, e.g. via social media: