Editing Mirror (section)

===Surface roughness===
[[Surface roughness]] describes the texture of the surface, often in terms of the depth of the microscopic scratches left by the polishing operations. Surface roughness determines how much of the reflection is specular and how much diffuses, controlling how clear or cloudy the image will be.

For perfectly specular reflection, the surface roughness must be kept smaller than the wavelength of the light. Microwaves, which sometimes have a wavelength greater than an inch (~25&nbsp;mm) can reflect specularly off a metal screen-door, continental ice-sheets, or desert sand, while visible light, having wavelengths of only a few hundred nanometers (a few hundred-thousandths of an inch), must meet a very smooth surface to produce specular reflection. For wavelengths that are approaching or are even shorter than the [[Atomic radius|diameter of the atoms]], such as [[X-ray]]s, specular reflection can only be produced by surfaces that are at a [[grazing incidence]] from the rays.

Surface roughness is typically measured in [[micron]]s, wavelength, or [[Sandpaper#Grit size table|grit size]], with ~80,000–100,000&nbsp;grit or ~½λ–¼λ being "optical quality".<ref name=duzg2011/><ref name=walk1998/><ref name=warn/>