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( Opacity (optics))
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). An opaque substance transmits very little light, and therefore reflects, scatters, or absorbs most 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 transparent in the visual range, are largely opaque to ultraviolet light. More extreme frequency-dependence is visible in the absorption lines of cold gases. In general, a material tends to emit light in the same proportions as it absorbs it. The opacity ?? gives the rate of absorption (or extinction), which is the fraction of the intensity I?, of the radiation that is absorbed or scattered per unit distance along a ray of propagation For a given medium it has a numerical value that may range between 0 and infinity. It is also called the absorption coefficient (see also extinction coefficient). In general ?? depends on the frequency ? of the radiation, as well as the density, temperature, and composition of the medium. The mean free path is the distance a photon travels in the medium before absorption or scattering is defined as 1 / (???), where ? is the density of the material. The notation ?? is the opacity described as a function of wavelength ?. While many materials are very opaque (steel in visible light having near-infinite opacity), and others very transparent (air in visible light having near-zero opacity), so that opacity often seems to be a boolean property, many others (such as water) have intermediate opacity. In astronomy and planetary imaging fields, tau, the optical depth, defines the opacity zero indicates transparent and higher numbers indicate more and more opaque in an inverse exponential fashion, for example a tau of 1 indicates 36 percent of the light passes (e-1 = 0.36), and a Tau of 5 indicates less than 1 percent passes (e -5 = 0.0067).[citation needed][1]
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