Extinction curves for selected dust models (frame a). Frame b shows the values of the FIR opacity spectral index. Frame c shows the FIR-NIR optical depth ratios τ(250 μm)/τ(J), where the cyan dashed line is drawn at τ(250 μm)/τ(J)=0.4 × 10⁻³ and the magenta dashed line at τ(250 μm)/τ(J)=1.6 × 10⁻³. The THEMIS2 models (MC to MCPI) have their default grain size distributions with a₀ ∼ 0.05 μm.