I finished my online E&M class today, and the prof told us to experiment with polarizers. While doing that, I found this: I put a piece of plastic between two polarizers, and found that at a certain angle between the two polarizers, a portion - though not all - of the plastic darkened. So I assumed that portion of the plastic sheet was a polarizer, for whatever reason. And I also assumed that a single polarizer, when used with this piece of plastic, would show darkening. But when I used a single polarizer, nothing happened, no matter how I rotated the polarizer. So that means the plastic actually is not a polarizer, right? So what was going on when it was between the two polarizers? This piece of plastic happens to be semi-transparent red gift wrap, if that helps.