text: Temporal frequency is a scalar quantity, not a vector.\n\nHowever, spatial frequencies can be vectors. We usually do not deal with the vector spatial frequencies however, but the vector wavenumber K=(Kx,Ky,Kz), where the components Kx=1/Lx, Ky=1/Ly, Kz=1/Lz and Lx is the wavelength in the x direction, Ly is the wavelength in the y direction and Lz is the wavelength in the z direction. Unfortunately, there is an ambiguity in this definition, since some people mulitiply this definition of wavenumber by 2*pi (see reference).\n\nOf course, in a vacuum, the wavelength L=Lx=Ly=Lz (no anisotropy) and c=f*L, where c is the speed of light and L is wavelength, so c=f/k.