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1. the state or quality of being opaque
2. an opaque object or substance
3. Physics Photog the ratio of the intensity of light incident on a medium, such as a photographic film, to that transmitted through the medium
4. Logic Philosophy the property of being an opaque context
opacity(oh-pass -ă-tee) A measure of the ability of a gaseous, solid, or liquid body to absorb radiation. It is the ratio of the total radiant energy received by the body to the total energy transmitted through it. For gaseous material of a particular chemical composition, opacity depends on both temperature and density. Various processes contribute to the opacity: energy absorption by electrons bound to an atom or ion, allowing them to jump to a higher level or to escape from the atom as free electrons (bound-bound or bound-free absorption); absorption by free electrons (free-free absorption); scattering of photone of radiation by free electrons or atoms (Compton scattering); absorption of γ-ray photons by ambient ‘gas' photons (photon-photon absorption). The negative hydrogen ion, H–, is a particularly important source of opacity in a star like the Sun, and the solar opacity is seen to drop rapidly to zero at the Sun's limb.
Quality of being impenetrable by light; not reflecting light, or transmitting light, neither transparent nor translucent.
The light flux incident upon a medium divided by the light flux transmitted by it.
The quality of being opaque, as the capacity of a paint to cover or obliterate a background over which it is applied.