Why does a twisted-nematic LCD appear dark when voltage is applied?
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.
In a twisted-nematic (TN) cell, the display appears dark when voltage is applied because the electric field disrupts the liquid crystal's ability to guide light.12
In the absence of voltage, the liquid crystal molecules are arranged in a 90-degree twisted configuration. This structure acts as a waveguide, rotating the polarization of incoming light by 90 degrees so that it aligns with the second, crossed polarizer, allowing the light to pass through and making the cell appear transparent.12
When a voltage (typically three to five volts) is applied, the liquid crystal molecules align themselves parallel to the electric field, perpendicular to the substrate plates. This orientation destroys the twisted state, meaning the liquid crystal can no longer rotate the polarization of the light. Consequently, the light remains blocked by the second polarizer, causing that portion of the display to appear dark.12