Schematic comparison of the two different cases (a) and (b) for the transition from small grains that absorb by volume to big, opaque grains that absorb by cross-section. Case (a) has an intermediate size interval where the grains are already big compared to the wavelength but still transparent. In case (b) the transition from small to opaque is direct. The values assumed in Eqs. (17), (18), and (28) are β_s = 1.6, A₀ = 0.63 mm⁻¹, λ₀ = 1 mm, n = 3, y = n².