Compute the minimum index of refraction of a 2-D corner-cube reflector in such a way that it retro-reflects (at least some) light incident on the face from any angle.
Compute the minimum index so that the rear face of the retro-reflector reflects all light incident on it from the interior that would hit both surfaces – in another words, the index so the rear surface doesn’t need a mirror coating to efficiently reflect all light that can be retro-reflected.