Linear polarization of the bright side of Iapetus versus wavelength, observed with FORS2 at the ESO VLT in April 2009 at phase angle 2.90°. In the and panels, the red lines represent the measurements corrected for the combined offset due to the chromatism of the retarder waveplate + the Wollaston prism, the instrumental polarization, the FORS2 instrument offset of the respective epoch, and transformed in to a system where the reference direction is the direction perpendicular to the plane identified by the Sun, the object, and the observer (the scattering plane). The null profiles, N_U (offseted to  + 0.25% for display purpuses) and N_Q are displayed in blue, superposed to the statistical error bars (in light blue). Ideally, null profiles should be centered about zero with a Gaussian distribution that has the same σ as the corresponding Stokes parameters. Therefore, any point of the null profiles that significantly exceeds the light blue error bar suggests that the spectral points in the corresponding Stokes profiles are unreliable. The right panels show the fraction of linear polarization and its position angle, together with their error bars. For symmetric reasons, we expect the position angle to be centred about 90° (as well as we expect to be centred about zero). In all panels, the black dotted lines show the total flux (in arbitrary units).