Contact lenses correct vision in the same way as glasses: they change the direction of light rays to correctly focus light on the retina. If you are nearsighted, light rays focus too early in your eye: they form a focal point in front of the retina instead of directly on it. Contact lenses and glasses correct myopia by diverging light rays, reducing the eye's focusing power. This moves the focal point of the eye back, onto the retina, where it belongs. If you are farsighted, your eye does not have adequate focusing power: light rays fail to form a focal point when they reach the retina. Contact lenses and glasses correct farsightedness by converging light rays, which increases the focusing power of the eye. This moves the focal point of the eye forward, onto the retina. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essay The powers of contact lenses and spectacle lenses are expressed in diopters (D). Powers of lenses that correct myopia begin with a minus sign (–), and powers of lenses that correct farsightedness begin with a plus sign (+). So why are contact lenses so much thinner than spectacle lenses? Why are contact lenses important. This is largely because contact lenses sit directly on the eye, instead of being about 12 millimeters from the surface of the eye like spectacle lenses. Due to their proximity to the eye, the optical zone of contact lenses (the central part of the lenses which contains the corrective power) can be made much smaller than the optical zone of spectacle lenses. Indeed, the optical zone of spectacle lenses is the entire surface of the lens. The optical zone of contact lenses is only a part of the lens, which is surrounded by peripheral adjustment curves that do not affect vision. It's a bit like looking out of a small window in your house: if you stand very close to the window, you have a wide, clear 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 sit 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). In 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. Keep in mind: this is just a sample. Get a personalized paper now from our expert writers. Get a Personalized Trial Additionally, eyeglass lenses must be much thicker than contact lenses to prevent them from breaking upon impact. Myopia lenses in eyeglasses must have a minimum center thickness of 1.0 mm or more 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 0.1mm. It is therefore the combination of significant differences in wearing position, optical zone diameter and minimum thickness to ensure structural integrity that makes contact lenses much more..