How do liquid crystal displays use electric voltage to control light?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
Liquid crystal displays (LCDs) control light by using an electric field to alter the orientation of liquid crystal molecules, which in turn changes the material's optical properties.12
In a common twisted-nematic cell, liquid crystals are sandwiched between two crossed polarizing filters. In their natural state, the molecules are arranged in a 90-degree twist, which rotates the polarization of incoming light, allowing it to pass through the second filter and appear transparent.34
When a small electric voltage is applied, the molecules realign with the electric field, effectively "untwisting" their structure. In this state, the crystals no longer rotate the light's polarization, preventing it from passing through the second filter and causing that area of the display to appear opaque or black. By applying this voltage to specific pixels in a grid, the display creates images through a patchwork of light and dark areas. This process is highly efficient, requiring very little electric power compared to other display technologies.342