Liquid crystal displays are probably one of the most commonly used devices based on light polarization. They work because liquid crystals are usually optically active but lose this property in the presence of an electric field.1 Simple displays have four layers (see Fig. 1): a /4 layer of liquid crystals (embedded as segments within glass and surrounded by electrodes) between two crossed polarizers and a reflector. If nothing is displayed, incoming light gets polarized at the top polarizer, the polarization is turned 90° in the liquid crystal layer, passes through the bottom polarizer, gets reflected, and takes the reverse route to the viewer. When a segment is to be darkened, an electric field is used to make the crystals optically inactive, and light cannot pass through the crossed polarizers.
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