  The investigation of polarized light of protoplanetary disks is key for
constraining the dust properties, disk morphology and embedded magnetic fields.
However, different polarization mechanisms and the diversity of dust grain
shapes and compositions lead to ambiguities in the polarization pattern. The
so-called "self-scattering" of thermal, re-emitted radiation in the infrared
and mm/submm is discussed as a major polarization mechanism. If the net flux of
the radiation field is in radial direction, it is commonly assumed that the
polarization pattern produced by scattering in a protoplanetary disk shows
concentric rings for disks seen in face-on orientation. We show that a flip of
$90^\circ$ of the polarization vectors may occur and mimic the typical pattern
of dichroic emission of dust grains aligned by a toroidal magnetic field in
disks seen close to face-on. Furthermore, this effect of polarization reversal
is a fast changing function of wavelength and grain size, and thus a powerful
tool to constrain grain composition and size distribution present in
protoplanetary disks. In addition, the effect may also provide unique
constraints for the disk inclination, especially if the disk is seen close to
face-on.
