Editing Wave vector (section)

==Direction of the wave vector==
{{Main|Group velocity}}
The direction in which the wave vector points must be distinguished from the "direction of [[wave propagation]]". The "direction of wave propagation" is the direction of a wave's energy flow, and the direction that a small [[wave packet]] will move, i.e. the direction of the [[group velocity]]. For light waves in vacuum, this is also the direction of the [[Poynting vector]]. On the other hand, the wave vector points in the direction of [[phase velocity]]. In other words, the wave vector points in the [[surface normal|normal direction]] to the [[Wave front|surfaces of constant phase]], also called [[wavefronts]].

In a [[attenuation|lossless]] [[isotropy|isotropic medium]] such as air, any gas, any liquid, [[amorphous solids]] (such as [[glass]]), and [[cubic crystal]]s, the direction of the wavevector is the same as the direction of wave propagation. If the medium is anisotropic, the wave vector in general points in directions other than that of the wave propagation. The wave vector is always perpendicular to surfaces of constant phase.

For example, when a wave travels through an [[anisotropy|anisotropic medium]], such as [[crystal optics|light waves through an asymmetric crystal]] or sound waves through a [[sedimentary rock]], the wave vector may not point exactly in the direction of wave propagation.<ref name=fowles>{{cite book|last=Fowles|first=Grant|title=Introduction to modern optics|year=1968|publisher=Holt, Rinehart, and Winston|page=177}}</ref><ref name=pollard>"This effect has been explained by Musgrave (1959) who has shown that the energy of an elastic wave in an anisotropic medium will not, in general, travel along the same path as the normal to the plane wavefront ...", ''Sound waves in solids'' by Pollard, 1977. [https://books.google.com/books?id=EOUNAQAAIAAJ link]</ref>