Contact lenses work to correct vision in the same way as glasses: they alter the direction of light rays to properly focus light on the retina. If you are nearsighted, the light rays focus too early inside the eye: they form a focal point in front of the retina rather than directly on it. Contact lenses and glasses correct myopia using divergent light rays, which reduce the eye's focusing power. This moves the eye's focal point backwards, onto the retina where it belongs. If you are farsighted, your eye does not have adequate focusing power: light rays cannot form a focal point by the time they reach the retina. Contact lenses and glasses correct hyperopia by converging light rays, which increases the eye's focusing power. This moves the eye's focal point forward, onto the retina. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an Original Essay The powers of contact lenses and eyeglass lenses are expressed in diopters (D). Lens powers that correct myopia begin with a minus (–) sign, while lens powers that correct farsightedness begin with a plus (+) sign. So why are contact lenses so much thinner than spectacle lenses? Why contact lenses are important. In large part, that's because contact lenses rest directly on the eye, rather than about 12 millimeters away from the surface of the eye like eyeglass lenses. Because of their proximity to the eye, the optical zone of contact lenses (the central part of the lenses that contains the corrective power) can be made much smaller than the optical zone of eyeglass lenses. In fact, the optical zone of eyeglass lenses is the entire surface of the lens. The optical zone of contact lenses is only a portion of the lens, which is surrounded by peripheral fitting curves that do not affect vision. It's a bit like looking out of a small window at home: if you're very close to the window, you have a wide, unobstructed view of the outside. But if you stand across the room from the window, your view of the outside is very limited, unless you have a much larger window. Since contact lenses rest directly on the cornea, their optical zone only needs to be approximately the same diameter as the eye's pupil in low-light conditions (about 9 millimeters). By comparison, to provide an adequate field of vision, most eyeglass lenses are larger than 46mm in diameter. This larger size makes eyeglass lenses much thicker than contact lenses. Please note: this is just an example. Get a custom paper from our expert writers now. Get a Custom Essay Additionally, eyeglass lenses need to be much thicker than contact lenses to prevent them from shattering on impact. Myopia lenses in eyeglasses must have a minimum center thickness of 1.0 mm or greater to meet impact resistance guidelines. Contact lenses can be much thinner. In fact, most soft contact lenses for myopia have a center thickness of less than 0.1 mm. So it's the combination of significant differences in wearing position, optical zone diameter, and minimum thickness to ensure structural integrity that makes contact lenses much, much thinner than spectacle lenses of the same power.