The wave vector (or k vector) of a plane wave is a vector which at least in case of isotropic optical media points in the direction in which the wave propagates. It is always perpendicular to the wavefronts. Its magnitude is the wavenumber as defined by
where λ is the wavelength in the medium (not the vacuum wavelength).
In non-isotropic media, the direction of energy flow, which is the direction of the Poynting vector, can somewhat deviate from that of the wave vector, which is always perpendicular to the wavefronts. This phenomenon is called spatial walk-off.
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