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lensesesOver 140 million people in the United States wear lenseses. ocular People whose eyes have refractive errors do not see clearly without health glasses, contact because the light emitted from the lens objects they are observing does not come into focus on their retinas. ocular For people who are farsighted, images come into focus behind the retina; for people who health are nearsighted, images come into focus contact in front of the retina. Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens and ocular lens material and the greater the difference in the curvature between health the two surfaces of the lens, the greater the change in direction of light that passes through it, and the greater the correction.Lenses can be unifocal, with one correction for all distances, or they can be correct for more than one distance contact and lens and ocular (multifocal). One type health of multifocal, contact the lens bifocal, has an area ocular of the lens health (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects contact and lens for distant objects (about 20 ft ocular from the eyes). Eye glasses and contact lenseses and contact are devices that correct refractive errors in vision. lenses lenses are mounted in frames health worn on the face, sitting mostly contact on the ears and nose, so that lens the lenses are positioned in front of the eyes. contact appear to be worn in direct contact with the cornea, but they actually float on a layer of tears that separates them from the ocular and health cornea.PurposeThe contact purpose of lenseses and contact is to correct or improve the vision of people lens and ocular and health with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism.Precautions People allergic to certain contact plastics should not wear contact or lenses frames or lenses manufactured from that type of plastic. People allergic to nickel should not wear Flexon frames. People at risk of lens being in accidents that might shatter glass lenses should wear plastic ocular lenses, preferably polycarbonate. Another type of multifocal, a trifocal, has health an area in-between that corrects for intermediate contact distances (usually about 28 in). Conventional lens bifocals and trifocals have visible lines between the areas of different correction; however, lenses where the correction gradually changes from one area to the other, without visible lines, have been available ocular since the 1970s. Such lenses are sometimes called health progressives or no-line bifocals.To be suitable for lenses lenses, a material must be transparent, without bubbles, and contact have a high index of refraction. The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact resistant than glass. The advantage of glass is that it is scratch resistant and lens provides the clearest possible vision.
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