In the last lecture we discussed how light bends, or refracts, when it goes from one medium to another. In this lecture we see how to exploit this refraction of light to cause parallel light rays to focus either to or from a point. These points are called focal points, and the object doing the refracting is  called a lens.  In our discussion, we will assume that the distance from one side of the lens to the other is much smaller than the focal length of the lens (the distance from the center of the lens to the focal point). Lenses satisfying this approximation are called thin lenses.