A liquid crystal display includes a light source for emitting light, a first substrate, a second substrate parallel to and facing the first substrate, and a plurality of pixel units formed between the first substrate and the second substrate. At least one pixel unit comprises a reflecting element disposed on the first substrate for reflecting light from the light source, and a photo-sensing element, formed on the second substrate, for outputting a sensing parameter based on light reflected from the reflecting member. Each reflecting element is extended out of the first substrate and faces to one of the plurality of photo-sensing elements. A position of the force applied on the first substrate is determined by detecting a variation of the sensing parameter outputted by the photo-sensing element.