# Dictionary:Wavenumber (k)

{{#category_index:W|wavenumber (*k*)}}
**1**. The number of waves per unit distance perpendicular to a wavefront, that is, the reciprocal of the wavelength. It equals , that is, wavenumber *k* is to as frequency *f* is to angular frequency . (Some authors define as the wavenumber.)

**2**. Spatial frequency, the number of wave cycles per unit of distance in a given direction (direction of the spread); **apparent wavenumber**. Specifically, the reciprocal of the apparent wavelength along the spread direction:

where *f*=frequency and *V*_{a} is apparent velocity. If a wavefront makes the angle with the given direction,

where *V* is the actual velocity of the wavefront. A wavenumber of zero indicates a wavefront striking a line of detectors simultaneously. See *f-k plot*.

**3**. See *propagation constant*.