https://doi.org/10.1351/goldbook.09441
Vector in reciprocal space describing the direction of propagation of X-rays and equal in magnitude to \(2\uppi/\lambda\), where \(\lambda\) is the wavelength of the X-rays.
Note: In different fields of study, the wave vector is taken as \(2\uppi/\lambda\) or as \(1/\lambda\). In X-ray reflectometry the former is usually used.