See how polarizing lenses can selectively block light.
Light normally bounces around in all directions, much like someone shaking a rope.
A polarizing filter acts like a picket fence, only allowing light bouncing in a single direction to pass. The procedure you followed with the clear plastic can be used for real stress testing. Scientists and engineers build models of buildings, bridges, etc. If you have a digital wristwatch that shows the time as little black numbers, you are using polarizing filters. Those numbers are created using Liquid Crystal Displays, or LCDs.