Transparency of a Medium

the ratio of the radiation flux (or, for visible light, of the luminous flux) traversing a unit path in the medium without a change of direction to the flux that entered the medium in the form of a parallel pencil. Thus, media with significant and basically direct transmission of optical radiation have a high transparency. In the region of visible light, objects are distinctly visible through bodies consisting of such media when the bodies have appropriate geometric shapes. In the general case, transparency is a function of the wavelength of the radiation; with respect to monochromatic light, the term “monochromatic transparency” is used.

Transparency is a measure of how small a role is played by the absorption and scattering of radiation in a medium; the effects of the interface between the bodies are not taken into consideration in defining their transparency. Transparency is distinguished from transmission in general, since a medium may be nontransparent but at the same time may transmit scattered light, for example, thin sheets of paper. Accordingly, transparency is related only to the transmission factor for direct, but not diffuse, transmission. In a layer 1 cm thick, the transparency of optical quartz is approximately 0.999, while that of optical glass is ˜0.99–0.995.