Polarisation spectrum of thermal dust polarisation towards a starless core for different values of the line-of-sight visual extinction ($A_{\mathrm{V}}^{\mathrm{l}\mathrm{o}\mathrm{ss}}$), shown for a_max = 0.25 μm (left panel) and a_max = 0.5 μm (right panel). The spectrum first increases rapidly and then increases gradually towards longer wavelengths. Higher a_max yields a higher degree of polarisation under the same physical condition (characterised by A_V).