Reflector sight
A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an aiming point or some image (helping to aim the device, to which the sight is attached, on the target) superimposed on the field of view. These sights work on the simple optical principle that anything (such as an illuminated reticle) at the focus of a lens or curved mirror will appear to be sitting in front of the viewer at infinity.