Refraction with a biconvex lens. Illustration showing plane (or straight) waves converging as a result of refraction by a biconvex lens (light blue). As the waves pass through the lens they slow down and are refracted due to the greater density of the lens material. Due to the biconvex shape of the lens, the waves are refracted causing them to converge. The more severe the convex shape, the more the waves converge.