Attenuation versus the mean optical depth at a given wavelength λ, for a face-on galaxy with a slab geometry (solid black line), and a clumpy geometry assuming a mean number of clouds on the line of sight of 1 (blue), 2 (green), 3 (red), or 4 (cyan). A simple screen geometry is also shown for reference (dashed black line). For simplicity, we have not taken into account the effect of the albedo in this plot; that is, we have assumed that ω_λ = 0.