Opacity

Synonyms

Attenuation

Definition

Opacity measures the property of a medium to attenuate light. The opacity depends on the composition of the medium, its density, and temperature, but also on the wavelength considered. The absorption coefficient  a_ν (with units [cm⁻¹]) enters into the definition of the optical depth. More often in astronomy, it is the mass absorption coefficient (with units [cm² g⁻¹]) which is used.