Use a transparent reciprocal <wave plate> at its design <wavelength>. For the return calculation, assume a mirror at <normal incidence> with the same reflection coefficient for both transverse components; this is the usual ideal <double-pass quarter-wave plate> arrangement.

Solution

Past exam of the optics and electrodynamics course of the University of Cambridge / 2019 / Section A / A2 / Solution

 Solution (source code)