7. Using the radiative transfer equation of a simple slab-like absorbing medium with no internal emission source, I = I0 exp(−n σ x), where I0 is the original photon flux, σ is the absorption cross section, n is the particle number density of medium, and x is the path length through the medium.
7a) What is the formula to calculate τ in terms of quantities that are provided above?
7b) What fraction of the original photon flux would be absorbed by the medium of τ = 5?