An optics demonstration that I saw many years ago involved two identical-looking transparent plastic "slides", about the size of microscope slides. Call them slides A and B. Individually, they were transparent to ordinary ambient light passing through them in either direction. However, when stacked together, the stack was transparent only when viewed from one side, and was opaque when viewed from the opposite side:
| | | | A | B | | | | -------------> 1 | | | | | | 0 <------------- | | |
I recall that this had to do with polarization, and at least one of the slides was a waveplate (quarter?, half?), but I can't figure out exactly what must have been used to produce this effect. Help, please?