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Fig. 14   (a) A schematic diagram showing layers of "rod-shaped" molecules in a liquid crystal. (b) As light passes through the liquid crystal, the polarization direction of the light (blue arrows) is being changed to match the molecules' orientation.

Liquid crystal is a substance half way between a solid and a liquid. It is formed from many layers of "rod-shaped" molecules (Fig. 14a). Within a layer, the molecules have the same orientation (i.e. the "rods" point in the same direction); but orientation differs across layers. As we move across layers, the orientation of the molecules in a layer differs from those of the previous layer by a fixed angle, so that there is a continuous "rotation" of orientation angles across the layers, making the whole stack of layers look like a twisted structure. Interestingly, when polarized light passes through a liquid crystal, the molecules will change the polarization direction to match their own angle (Fig. 14b). So by letting polarized light pass through a liquid crystal of a given thickness, we can effectively rotate the polarization direction of the light wave by a certain angle.

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There are many small display units (called pixels) on a colour LCD panel. In each unit, a layer of liquid crystal is sandwiched between two polarizers (Fig. 15). Light produced behind the panel is first polarized by the first polarizer. As the polarized light travels through the liquid crystal, the molecules begin to change the light's polarization direction. The molecule arrangement is "twisted" 90 degrees as shown in Fig. 16a. As a result, as light comes out of the liquid crystal, its polarization direction is rotated by 90 degrees. This polarization direction matches the orientation of the second polarizer, and thus light coming out from the liquid crystal can pass through the second polarizer.

Fig. 15   The display unit of an LCD monitor. A layer of liquid crystal is sandwiched between two polarizers. The voltage applied to the liquid crystal controls the passage of light through the second polarizer (see Fig. 16). The colour displayed is controlled by the relative intensity of light passing through the red, green and blue filters. Fig. 16   (a) When no voltage is applied to the liquid crystal, light can pass through the combination of polarizers and liquid crystal, (b) when a high enough voltage is applied, the liquid crystal 'untwists' completely and no light can pass through the combination.
 
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When a high enough voltage is applied to the liquid crystal, the layers of molecules will "untwist" so that molecules of all layers now have the same orientation direction. As a result, the light's polarization direction is no longer changed as it passes through the liquid crystal. Light coming out of the liquid crystal is polarized in a direction perpendicular to the orientation of the second polarizer, so it cannot pass through the second polarizer (Fig. 16b). If a lower voltage is applied to the liquid crystal, it "untwists" less and part of the light can pass through the second polarizer. Hence by adjusting the voltage applied to the liquid crystal, we can control the amount of light passing through this combination of liquid crystal and polarizers.

As shown in Fig. 15, each display unit of the LCD panel has red, green and blue colour filters at the light outlet. The combination of liquid crystal and polarizer described above is used to control the intensity of light passing through each of these colour filters. Similar to a CRT monitor, the colour of a display unit is controlled by the relative intensity of red, green and blue light. By combining the colour light emitted by many display units, a colour image is formed on the screen.

LCD monitors use much less energy than CRT monitors as LCD monitors do not have to produce an electron beam. Most of the energy supplied to LCD monitors is used by fluorescent tubes which form the light source of the LCD display units.

The following animations will show you more details about the operation of a CRT monitor and an LCD monitor.

Flash animation: 1.  Operation of a CRT monitor   2.  Operation of an LCD monitor