Past exam of the optics and electrodynamics course of the University of Cambridge / 2013 / Paper 1 / 3 / b

With A nearest the observer, the outward order is P then Q. The initially unpolarized light becomes x linear, then circular, before reaching the mirror. Reflection reverses its helicity. Its return through Q gives y linear, which is blocked by P:Hence the ideal reflected filter is dark and the target cannot be seen through it. This is a double-pass polarization rejection effect, not unequal one-pass transmission from its two sides. Real mirrors at oblique incidence, imperfect retardance, finite bandwidth and depolarization can leave leakage.