Number distribution of rays through a standard core as a function of the optical depth for absorption (red), scattering (pink), and extinction (black), respectively. The thin and thick lines correspond to MRN(0.25) and MRN(1) distributions, respectively. The dash-dotted lines give the limiting τ for which 5% of all rays have larger τ. The three vertical bars in the upper left corner indicate the τ values for which the relative error in the Taylor expansion of the extinction in first and second order reaches 10%, and the τ where the probability for second scattering reaches 10%.