= Solution
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:
$$
\boxed{J_{\rm return}=PQM QP=-PQ^2P=0.}
$$
Hence \b[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.
Back to article page
