Past exam of the optics and electrodynamics course of the University of Cambridge / 2022 / Section B / 4 / e / 2 / Solution

Keep source, observer and magnetic field in their original places, but interchange the physical polarizers, including their pass-axis orientations. The forward order is then $P_2$, glass, $P_1$, so the new Jones matrix is again $P_{1}R(\theta )P_2$. ConsequentlyThis is the same intensity change as exchanging source and observer. At the field that maximizes the original forward transmission, the swapped arrangement blocks the beam. If both operations were performed together, the original order relative to propagation would be restored and so would the original transmission. The answer concerns exchanging the two differently oriented pass axes; simply relabelling two identical polarizers while leaving their axes fixed would not change the apparatus.