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When two polarizing filters are placed atop one another, they can be transparent or opaque to light. By rotating one of the filters, the transmitted light passing through the filters may be turned 'on' or 'off'. When the filters do not transmit light, the polarizing filters are said to be 'crossed polarizers'. Certain materials such as cellophane tape, Plexiglas, corn syrup, and stretched polyethylene exhibit beautiful colors when placed between two crossed polarizing filters.
About Polarizers: Only vertically oriented light waves may pass through the polarizing filter on the left. Only horizontally oriented light waves may pass through the filter on the right. If the filter on the left is placed on top of the filter on the right, no light will be able to pass through at all.
If the polarizing filters are aligned parallel to each other, light may pass freely through both filters. By placing transparent objects between two polarizing filters, it is possible to identify those materials which rotate polarized light!
Try sandwiching a plastic baggie between two filters and stretching it. When certain plastics are put under stress, they rotate polarized light. Try placing transparent tape between two polarizing filters. Some brands of tape work better than others. The more layers of tape, the more light is rotated.
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