PX282 - C6b - intensity after absorption

\frac{d I_{λ}}{d z} = - κ_{λ} ρ I_{λ}

\begin{aligned} \int_{I_{λ 0}}^{I_{λ} (z)} \frac{1}{I_{λ}} d I_{λ} & = \int_{0}^{z} - κ_{λ} ρ d z \\ [\ln I_{λ}]_{I_{λ, 0}}^{I_{λ} (z)} & = \int_{0}^{z} - κ_{λ} ρ d z \\ \frac{I_{λ}}{I_{λ, 0}} & = \exp (\int_{0}^{z} - κ_{λ} ρ d z) \end{aligned}

∴ I_{λ} (z) = I_{λ, 0} \exp (- κ_{λ} ρ z)