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Practical Uses of Polarized Light
 Stress Analysis Certain plastics rotate the plane of polarization of light passing through them. The angle through which the plane of polarization is rotated depends on the wavelength of the light. The angle is also found to vary when the sample is placed under stress, for example by bending it. This photograph shows a small part of a plastic set square viewed under normal conditions. The next photo shows the appearance of the same plastic object when placed between two polarizing filters with their planes of polarization at 90° to each other (we describe this situations as "two crossed polarizers"). Remember that two crossed polarizers would normally not let any light through but as the plastic material rotates the plane of polarization, some light is transmitted. As the angle of rotation depends on the stress in the material and the wavelength of light, we see different colours in different places. At places where the internal stresses are greatest, the coloured bands change more rapidly (with distance). Engineers wishing to predict where a mechanical component might fail, when placed under stress, can make a model of the component out of, for example, Perspex and observe the stress patterns (the coloured bands are more concentrated where the stresses are greatest. The design of the model can then be modified accordingly before the actual component is manufactured. Measuring the concentration of solutions Certain solutions rotate the plane of polarization of light passing through them. The angle through which the plane of polarization is rotated depends on the concentration of the solution. Liquid Crystal Displays LCDs are used in a great number of devices from watches and clocks to computer screens to HDTV screens... On the under-side of the top plate of the liquid crystal container, A, are fine lines etched parallel to the plane of polarization of the top Polaroid. Similarly there are lines etched into face B parallel to the plane of the lower Polaroid. The liquid crystals line up with these fine lines but also tend to line up with each other so there is a gradual change in the alignment of the crystals from A to B. The crystals change the plane of polarization of the light so that the light can pass through the lower polarizer and be reflected by the mirror: the display appears light. When a voltage is applied, the crystals line up along the direction of the electric field and they therefore no longer allow light to pass through: the display appears dark.
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