Light travels faster in a rare than in a dense substance. Therefore when a ray passes from a rarer to a denser substance, it is bent on entering and on leaving the denser substance, and in both cases the refraction or bending is toward its base (Fig. 64). When light passes from the air through water or a prism, the rays are bent. This fact is taken advantage of by manufacturers and others who are located in thickly settled communities, where the streets are narrow and the buildings are high. The upper panes of the windows are then made of a peculiar combination of prisms and lenses. By means of this device, the rays of sunlight in the street or yard are deflected from their original direction and projected and diffused into the stores, rooms, and basements. All forms of prismatic glass reflect the rays of light downward.