text: is the frequency of light a vector? relativity theory temporal frequency is a scalar quantity, not a vector.  however, 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).  of 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.