Smooth on the scale of the wavelength of the electromagnetic radiation of interest. No surface is absolutely smooth, if for no other reason than matter is composed of molecules in motion. An approximate criterion for smoothness is the Rayleigh criterion. A surface is reckoned to be optically smooth if d < λ/(8 cos θ), where d is the surface roughness (e. G. , root-mean- square roughness height measured from a reference plane), λ is the wavelength of the incident illumination, and θ is the angle of incidence of this illumination. Thus, a surface that is smooth at some wavelengths is rough at others, or that is rough at some angles of incidence and smooth at others (e. G. , near-grazing angles).