Opacity (optics) - Quantitative Definition

Quantitative Definition

See also: Extinction (astronomy) and attenuation coefficient

The words "opacity" and "opaque" are often used as colloquial terms for objects or media with the properties described above. However, there is also a specific, quantitative definition of "opacity", used in astronomy, plasma physics, and other fields, given here.

In this use, "opacity" is another term for the mass attenuation coefficient (or, depending on context, mass absorption coefficient, the difference is described here) at a particular frequency of electromagnetic radiation.

More specifically, if a beam of light with frequency travels through a medium with opacity and mass density, both constant, then the intensity will be reduced with distance x according to the formula

where

• x is the distance the light has traveled through the medium
• is the intensity of light remaining at distance x
• is the initial intensity of light, at

For a given medium at a given frequency, the opacity has a numerical value that may range between 0 and infinity, with units of length2/mass.