From AMS Glossary
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).