# Polarization of clear plastic - a puzzle

• alanf
In summary, during an experiment with polarizers, the individual found that a piece of plastic between two polarizers caused a portion of the plastic to darken at a certain angle. They assumed this meant the plastic was a polarizer, but when using a single polarizer, no darkening occurred. It was later discovered that the plastic was not a polarizer, but rather a semi-transparent red gift wrap that exhibited polarization effects due to being a biaxial material. Further research on wave retarders was suggested.

#### 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.

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)?

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.

## 1. What causes clear plastic to become polarized?

The polarization of clear plastic is caused by the alignment of its molecules in a specific direction. This alignment occurs when the plastic is stretched or heated, causing the molecules to become polarized and allowing them to filter light in a particular direction.

## 2. How can I tell if a clear plastic object is polarized?

You can easily test if a clear plastic object is polarized by placing it in front of a polarized light source, such as a polarized sunglasses lens. If the object appears darker or changes color when rotated, it is polarized.

## 3. Can the polarization of clear plastic be reversed?

Yes, the polarization of clear plastic can be reversed by heating or stretching the plastic in the opposite direction. This will cause the molecules to realign in the new direction and change the polarization of the plastic.

## 4. What are some practical uses of polarized clear plastic?

Polarized clear plastic has many practical uses, including in polarized sunglasses, LCD screens, and optical filters. It can also be used in photography to reduce glare and improve image clarity.

## 5. Can clear plastic be polarized without stretching or heating it?

No, stretching or heating is necessary to polarize clear plastic. However, some plastic materials may already have some level of polarization due to their manufacturing process, but this can be enhanced by stretching or heating.