Velocity of Light | previous | next | feedback |
While the velocity of some particle is a quantity which is based on a fairly simple and unambiguous concept, the velocity of light (as of other wave phenomena) is a much more sophisticated matter. There are different kinds of velocities, which are quite different conceptually and can (particularly for light propagation in media) have substantially different values:
- The phase velocity is the velocity with which phase fronts propagate.
- The group velocity determines the speed with which intensity maxima propagate (e.g. the peaks of pulses).
- The velocity of information transport can differ from both phase and group velocity, see the article on causality.
Figure 1 illustrates the different velocities. In that example, the phase velocities of different frequency components vary linearly with frequency: the wavefronts of the higher-frequency components (drawn at higher positions) travel more slowly. The pulse maximum forms where the wavefronts coincide, and propagates with the (lower) group velocity. More details are given in the article on group velocity.
Figure 1: Propagation of a light pulse in a dispersive medium. Note that the phase fronts of different frequency components propagate with different velocities, and the pulse propagates with the group velocity, which is lower than all the phase velocities.
Further complications can arise from light propagation in inhomogeneous media, particularly in waveguides.
In vacuum, phase and group velocity (for plane waves) are identical at c = 299792458 m/s. Within the International System of Units (SI), the vacuum velocity of light has been defined to exactly match this value, which together with the definition of the second (via a hyperfine transition of cesium atoms) determines the length of a meter.
In some situations, the phase velocity or even the group velocity of light can exceed the vacuum velocity of light (→ superluminal transmission, "fast light"), although this is not associated with a violation of causality. There are other situations, associated with absorption or gain resonances, where the group velocity of light at least within a narrow spectral region is reduced (→ slow light). Enormous velocity reductions can be observed for narrowband resonances, as occur e.g. in ultracold gases.
The perhaps most peculiar effect is the occurrence of negative group velocities in situations with strongly negative dn / / dω.
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See also: phase velocity, group velocity, causality, superluminal transmission
Categories: general optics, physical foundations
