A liquid crystal display includes a main body, a screen positioned on the main body, a backlight module positioned in the main body and supplying light for the screen, a controller for controlling the brightness of the backlight module and an ambient light sensor positioned on the main body. The ambient light sensor includes a barrel, an optical sheet positioned at an end of the barrel and a photosensitive unit positioned at the other end of the barrel. The photosensitive unit creates a signal according to the ambient light received via the optical sheet, and sends the signal to the controller. The controller adjusts the brightness of the backlight module. The optical sheet includes a first surface, a second surface opposite the first surface and a plurality of V-shaped micro structures formed on the first surface.