2.3: Absorption, Scattering and Attenuation Coefficients
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The decrease in radiance −dL as a beam of radiance L passes through a medium of thickness ds as a result of absorption is
−dL=αLds
where α is the linear absorption coefficient. With similar equations we can define the linear scattering coefficient σ and the linear attenuation (extinction) coefficient ε. The SI units of α, σ and ε are m-1 and ε=σ+α.
The mass absorption coefficient, mass scattering coefficient and mass extinction coefficient each with units m2 kg-1 are defined respectively as α/ρ, σ/ρ and ε/ρ, where ρ is the density (kg m-3) of the medium. Chandrasekhar uses κ for the mass extinction coefficient, which, in the theory of stellar atmospheres, is also known as the opacity.
The atomic (or molecular) absorption, scattering and extinction coefficients are respectively α/N, σ/N and ε/N, where N is the number density (atoms or molecules per unit volume), with units of m2/atom (or molecule). Because of these units the coefficients are often referred to as cross-sections.