Size and radial distributions of particles in disc models FG (left column) and SW1 (right column). Top: normal optical thickness per size decade τ_⊥ as a function of grain size s. The curves depict the contributions at several distances from the star. For comparison, the straight line in panel a) shows the theoretically predicted slope for an infinite collisional cascade assuming with b_s = − 0.37, which is n(s) ∝ s^-3.7 (O’Brien & Greenberg 2003). Bottom: radial profiles of the total optical thickness (integrated over all grain sizes).