The major difference between concave and convex lenses lies in the fact that concave lenses are thicker at the edges and convex lenses are thicker in the middle. These distinctions in shape result in the differences in which light rays bend when striking the lenses.
The concave lenses curve inward and bend light rays outward, causing the light rays to diverge. They are used in the treatment of myopia, or near-sightedness. Because light is focused in front of the retina in a near-sighted individual, the concave lenses help by allowing the light to bend and focus further back, thus promoting clearer vision at a distance. Concave lens are used in movie projectors to cause the image to spread. The lens of a normal eye is a double convex lens. Convex lenses round outward and cause light rays to converge. Convex lenses are beneficial in treating presbyopia, or far-sightedness. In a far-sighted person, the light is focused behind the retina. The convex lens corrects this problem and allows the person to see clearly up close. Convex lenses are also found in magnifying glasses, binoculars, telescopes and cameras. Certain telescopes and microscopes use concave mirrors as well as convex lenses. The convex lens of a human eye actually focuses the image upside down on the retina, but the brain interprets it the right way.