Polarization – Polarizers are used to do stress analysis in transparent plastics.  As light passes through the transparent plastic, each color of visible light is polarized with its own orientation. If the piece of plastic is placed between two polarizing filters, a pattern of colors appears. As you rotate the top plate, the color pattern changes as new colors become blocked and the formerly blocked colors are allowed through. This is easy to demonstrate in class by placing a plastic protractor between two Polaroid plates and placing them on top of an overhead projector. You can also take a piece of clear plastic newspaper wrapper between the two polarizers.  If you pull on the newspaper wrapper to create stress, you will see the colors appear.  It is known that structural stress in plastic is signified at locations where there is a large concentration of bright colored bands. This location of stress is usually the location of weakness where structural failure is most likely to occur.