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Polarising filters can be used to create interesting and unusual special effects. The technique involves placing a polarisation filter over the lens of a camera, and also a devising a constant polarised light source. The latter can be simply created by fitting a second polarising filter over a normal light source, or using a lightbox covered by a sheet of polarising gel. A slide projector can also be used but flash is more difficult to handle.

A transparent plastic item, typically a perspex protractor or set-square from a geometry set, is then backlit using the polarised source. Other objects such as polythene bags and pieces of transparent plastic sheet can also be used. Stress patterns within the plastic are then revealed as abstract designs consisting of vivid colours.

Having set up a camera and appropriate lens on a tripod, switch on the backlighting and view the subject through the lens and its polarising filter. Rotate the polariser on the lens to see how the colours and patterns change with the various angles of cross polarisation. Use a small aperture such as f/8 or f/11 to give good depth of field ,and bracket exposures a little either side of the metered value. Areas of the image not occupied by the plastic objects are rendered black.

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