Some kinds of liquid crystals can rotate the plane of polarized light and this property is utilized in digital display unit used in watches, calculators, digital diaries and cameras.
The display unit is made-up of a thin film of the liquid crystal 
sandwiched between two thin glass sheets coated with a transparent conducting material such as tin oxide which acts as the electrodes. On the top and bottom of the glass sheets are placed two polarizing filters or polarizers turned 90° with respect to each other. Since in the liquid crystal film the polarized light is entering from the top by 90°, the light can pass through the second polarizer also, and when reflected from a mirror placed below the second polarizer, the display panel appears clear. When a small voltage is applied across one or more electrodes which are laid on the glass in the shape of alphabets, digits or symbols, the molecular alignment in the respective regions of the liquid crystal film is altered and they no more rotate the polarized light. This makes the characters on the display unit appear black on a clear background. Liquid crystal displays are quick acting and consume negligible power.
