Mirror is a reflecting surface that forms an image of an object when light rays coming from that object fall upon it. Usually mirrors are made of plate glass, one side of which is coated with metal or some special preparation to serve as a reflecting surface. The junction of this reflecting surface and the plate glass is called the mirror line. Highly polished metal and other materials serve also as mirrors, fused silica is used for applications that require high precision because of its very low thermal expansion.