Polarisation spectrum p_ext/AV vs. λ of starlight dust polarisation for different lines of sight (A_V), shown for maximum grain sizes of 0.25 μm (left panel) and 0.5 μm (right panel). For λ ≤ 10 μm, the spectral feature is broader for larger maximum grain size. When A_V is low, smaller a_max yields a greater amplitude; however, this relationship inverts as A_V increases. The oblate grains with an axial ratio of 1.4 are adopted.