 Solution

With B nearest the observer, light from the reflecting target near the observer first crosses Q and then P on its way to the mirror. Unpolarized light remains unpolarized through Q, then becomes x linear after P. Mirror reflection preserves that linear orientation. On return it passes the same P without further ideal rejection and then Q, becoming circular toward the observer. Thus the reflected target remains visible.

Algebraically, with input and output at the observer side,The target here is used only as a source/reflector of ordinary unpolarized light; no physiological model is involved.