P_r in the negative polarisation branch of the modelled dust aggregate having porosities of 60, 80, and 95% at the wavelengths of (a) 7000 Å, (b) 8000 Å, and (c) 9000 Å, and (d) the spectral dependence of P_r over the three wavelength domains. The results represent the mean P_r values and their standard deviations over the 1° –10° phase angle interval. Black (grey) symbols indicate the modelled results of the dust in the composition of 75 (65) vol.% amorphous carbon +25 (35) vol.% silicate. A more detailed description of the modelling is provided in the text. The P_r of the transparent dust at 9000 Å is −3.94 % and is not shown in this figure.