Image formation by a convex lens. Diagram showing the refraction of light beams through a biconvex lens producing an inverted image on a screen. 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. It produces a sharp inverted image of an object at a point behind the lens, called the focal point. Biconvex lenses are used to correct hypermetropia (long-sightedness), a condition in which light is focused behind the retina.