PX282 - C8 - optical depth

d τ_{λ} = κ_{λ} ρ d z

τ_{λ} = \int_{0}^{z} κ_{λ} ρ d z

\frac{I}{I_{o}} = e^{- τ_{λ}}

for $τ = 0, I = I_{0} ⟹$ no absorption
as $τ$ increases, $I$ decreases
so, the optical depth, $τ$ , is higher in a more absorbing material
in stars, and observations, the optical depth is a measure of how far into a material can be seen
therefore, it can be said that the surface of the sun is at a constant $τ$ , say $τ = 1$
if $κ$ is low, it means that the material is more transparent, and if it is high, then the material is more opaque