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(February 2010)| Opacity is the measure of impenetrability to
electromagnetic or other kinds of radiation, especially visible
light. In radiative transfer, it describes the absorption and
scattering of radiation in a medium, such as a plasma,
dielectric, shielding material, glass, etc. An opaque object is
neither transparent (allowing all light to pass through) nor
translucent (allowing some light to pass through). When light
strikes an interface between two substances, in general some may
be reflected, some absorbed, some scattered, and the rest
transmitted (also see refraction). Reflection can be diffuse,
for example light reflecting off a white wall, or specular, for
example light reflecting off a mirror. An opaque substance
transmits no light, and therefore reflects, scatters, or absorbs
all of it. Both mirrors and carbon black are opaque. Opacity
depends on the frequency of the light being considered. For
instance, some kinds of glass, while tra