Lensing and asymmetry of Schwarzschild QU loops. The left panel depicts the hot spot seen at zero inclination, and the right one is edge-on. The green arrows show the wave vectors k in Minkowski spacetime that connect the hot spot to the observer. The blue arrows show the corresponding wave vectors for the Schwarzschild case. They differ from Minkowski due to light bending, which adds a shift to the wave vector, depicted in pink. This shift vector is constant with the orbital phase and along the positive radial direction at zero inclination. However, it varies a lot with the orbital phase for the edge-on view, from zero at the closest point to the observer to purely vertical at the furthest point (i.e., “on the other side of the black hole”). The difference in dependence of the shift vector with the orbital phase as a result of inclination has a considerable impact on the Schwarzschild QU loop asymmetry (see text for details).