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Why do LCDs need a backlight or another source of illumination?

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liquid crystal Figure 1: Arrangements of molecules.

Liquid crystal displays (LCDs) require a backlight or an external source of illumination because they do not emit their own light.1

The fundamental operation of an LCD relies on the liquid crystal layer acting as an electro-optical shutter. By applying an electric voltage, the orientation of the liquid crystal molecules changes, which in turn alters how they manipulate the polarization of light passing through the device. Because the liquid crystal layer itself is passive and only serves to block or transmit light by interacting with polarizing filters, an external light source is necessary to provide the illumination that the display then modulates to create an image.234156

While most LCDs use a backlight, some devices are designed as reflective displays, which utilize ambient light reflected by a mirror to illuminate the screen, thereby reducing power consumption.6