How do liquid crystals produce optical effects used in watches and calculator displays?

Liquid crystals are substances that possess both the flow properties of liquids and the ordered molecular structures of crystalline solids. Their use in devices like watches and calculators relies on electro-optical effects, where an electric field is used to alter the orientation of the molecules, thereby changing the material's optical properties.12

Most common displays utilize a twisted-nematic cell. In this setup, liquid crystal molecules are sandwiched between two crossed polarizers. In their natural state, the molecules are aligned in a 90° twist, which rotates the polarization of incoming light, allowing it to pass through the assembly and appear transparent.34

When a voltage is applied, the molecules reorient to align with the electric field, effectively "untwisting" the structure. This prevents the rotation of light polarization, causing the light to be blocked by the second polarizer and making the cell appear dark. By patterning electrodes into segments, these devices can selectively darken specific areas to form numbers or letters against a reflective background.34