# What components would produce this polarization/waveplate demo? [duplicate]

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?

• By 'transparent', do you mean fully transparent, or with some absorbance (say, around the level of common sunglasses)? How does it compare, say, to the pictures here? – Emilio Pisanty Apr 21 '19 at 18:15
• @EmilioPisanty - As, I recall, they were individually fully transparent, practically indistingushable from ordinary microscope slides (except perhaps a bit thicker). – r.e.s. Apr 21 '19 at 18:18
• Thanks for clarifying. The configuration makes no sense to me. – Emilio Pisanty Apr 21 '19 at 18:18
• @EmilioPisanty - Thanks very much for that link. The effect is so similar to what is described there, I think I must have simply misremembered the demo. (Sorry to have cluttered the site with this, my first question. Can it be simply deleted?) – r.e.s. Apr 21 '19 at 18:31
• You can delete it if you really feel you need to, but if that's what you meant then the best course of action is to close it as a duplicate - you should see a prompt on the question now, and clicking "that solved my problem" will close it automatically. – Emilio Pisanty Apr 21 '19 at 18:33