Suppose a gas has an absorption cross section per molecule () for light of frequency (see 18.2 for an introduction to cross sections). Show

Suppose a gas has an absorption cross section per molecule σ(ω) for light of frequency ω (see §18.2 for an introduction to cross sections). Show that the gas’s absorption coefficient is given by κ(ω) =σ(ω)ρ, where ρ is the number density of gas molecules.