Refraction by a convex lens. Diagram showing the refraction of light beams through a biconvex lens (convex on both sides). Light changes direction, or is refracted, when it meets a different transparent medium such as a lens. A biconvex lens causes light beams to converge at a focal point, F. This type of lens is used to correct hypermetropia (long-sightedness), a condition in which light is focused behind the retina.