Figure 3-16 Refraction Through Uniform and Variable-Thickness Glasses (a) Refraction is the change in direction of a light ray when it passes into or out of a transparent medium such as glass. A light ray that enters a denser medium, such as moving from air into water or glass, is bent or refracted to an angle more perpendicular to the surface than the angle at which it was originally traveling. If the glass is flat, then the light leaving it is refracted back to the direction it had before entering the glass. There is no overall change in the direction in which the light travels. (b) If the glass is in the shape of a suitable convex lens, parallel light rays converge to a focus at the focal point. As with parabolic mirrors, the distance from the lens to the focal point is called the focal length of the lens. (c) The straw as seen through the side of the liquid is magnified and offset from the straw above the liquid because the liquid is given a curved shape by the side of the glass. The straw, as seen through the top of the liquid, is refracted but does not appear magnified because the surface of the water is flat and the beaker has uniform thickness.