Extinction curves used for Model I, where the mass extinction coefficient is given by κ_ext(λ) = κ_abs(λ) + κ_sca(λ). Solid lines are for mass absorption coefficients κ_abs and dashes are for the scattering, κ_sca. The curves are based on the optical constants provided by Jaeger et al. (1994) and by Dorschner et al. (1995) for two grain populations. The adopted grain size distributions have an exponent p = 3.5, and minimal sizes are a_min = 0.03μm for both. The largest sizes are a_max = 3μm (blue) and a_max = 300μm (red).