Polarization of clear plastic - a puzzle

[alanf]

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.

 

[Andy Resnick]

Plastic can exhibit a wide variety of polarization effects: overhead transparency sheets are biaxial materials, most bulk plastic exhibits photorefractive effects, a single layer of Scotch tape is nearly a 1/8 wave retarder, IIRC:

http://www.quekett.org/resources/article-archive/magic-polarised-light
http://www.lhup.edu/~dsimanek/14/polaroid.htm

Your piece of plastic could have acted as a (somewhat) uniform retardation plate. What happens if you fold it on itself (to double the thickness)?

 

[alanf]

Thanks Andy. So it sounds like I need to read up on wave retarders. I'll take a look at the plastic again later today.