Grain size exponent p, defined through dn(a) ∝ a^pda for spherical grains of size a, as a function of the grain opacity exponent β for the wavelength range 200 μm to 1.3 mm, and where κ_ν ∝ ν^β. The curves for three different materials are shown, viz. for amorphous ice (H₂O, blue: Bertie et al. 1968), amorphous silicates (MgFeSiO₄, red: Jaeger et al. 1994) and amorphous carbon (C, green: Blanco et al. 1991). The values of β determined from observations are shown as dashed lines, referring to the sources of Table 3 and identified to the right.