Light from the back plane passes through the lower polarizer, enters the array substrate, and reaches the liquid crystal.

(a) Unbiased, the liquid crystal molecules aligns with the direction of the light. Light appears through the upper color filter (CF) substrate and the upper polarizer, and hence the “pixel” brightens up, or turned on.

(b) Once an electrical bias is applied, the liquid crystal alignment is disrupted and hence the light through the CF and the upper polarizer is attenuated. The electrical bias controls the degree of LCD molecule alignment. The extreme case is “off” or darkened.