How works a beam?

Yeah, from dust or it is Mie scattering. You know this because fog and dust looks white and not blue, therefore it isnt rayleigh scattering which has like a wavelength^-4 dependence. Pieces of dust and/or fog are on the order of microns typically which is bigger than the wavelength of the light in question so it scatters essentially the same for all colors that are visible.