Liquid crystals have the form of a fluid material midway between a crystalline and liquid state. Their molecules change directions when an electric current runs through them. This property can be used to block light or to let it through. Here, liquid crystals are sandwiched between two sheets of glass. When there is no current, the glass is opaque. When a current is applied, the liquid crystals all move in the same direction and let the light through so that the glass become transparent. This procedure has applications in the manufacture of windows for cars and buildings.
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