Lenses We have seen that when a ray enter and leave a transparent material through parallel sides, the exit and entrance rays are parallel to each other. Conversely, if the sides of the material where a ray enters and leaves are not parallel, then the direction of the exit ray is different from the entrance ray.
Figure la illustrates two rays passing through a prism.
Their direction of propagation has been changed as a result of the refraction that takes place in passing through the material with non-parallel sides. Figure 1b shows a slightly different situation. It s important to note that in both cases the rays are bent toward the base or thicker portion of the prism. In the latter case the rays appear to diverge from S' because they a are not parallel rays. Now we can use our knowledge of how the shape of a transparent medium changes the direction in which a light beam travels to design transparent materials specifically for redirecting light.