Place the lit bulb parallel to the surface of the plastic. Orient the bulb and your eye so you can see the reflection of the bulb in the plastic.
Look at the reflection through a piece of polarizer. Rotate the polarizer and vary the angle at which you look at the plastic until you get the dimmest reflection. You’ll probably get the best results when there’s about a 35-degree angle between your eyes and the piece of plastic (click to enlarge drawing below). Rotate the polarizer 90 degrees as you watch the reflection. The reflection should become notably brighter.
Observe reflections elsewhere around you. Rotate the polarizer and change the angle of viewing to vary the brightness. Try looking at a reflection from a metallic surface, such as an ordinary mirror. There should be no difference in the brightness of an image reflected in the mirror as you rotate the polarizer or vary the angle of viewing.
On a sunny day, look at the sky through the polarizing lens. Notice that in some places the brightness of the blue sky changes as you rotate the polarizer. That’s because the light in the sky is polarized.
Look through a polarizer at the surface of a pond on a bright, sunny day. Rotate the polarizer and notice that at one orientation of the polarizer, the surface reflections are greatly reduced and you can see beneath the surface of the water. Rotate the polarizer 90 degrees from this orientation, and the surface reflections block your view of the underwater world. This is why people wear polarizing sunglasses when they go fishing.