Top: dust polarization efficiency (C_2,abs − C_1,abs)/(C_2,abs + C_1,abs) as a function of the size parameter x = 2πa/λ for individual grains with a size comprised between 10 and 1000 μmm. We note that the x axis has been reverted for an easier comparison with the bottom panel. Bottom: same as a function of the wavelength λ, for 3 mean grain sizes a_peak of 25, 100, and 500 μm. The grain of oblate shape has an axis ratio b/a = 1.1 and is perfectly aligned with its direction of alignment in the plane of the sky (γ = 0^°).