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Eye glasses and contact lenseses and contact are devices that correct refractive errors in vision. lenses lenses are ocularsciencecontactlens mounted in frames worn on the face, sitting mostly on the ears and nose, so that 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 cornea.PurposeThe purpose of lenseses and contact is to correct or improve the vision of people with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism.Precautions People allergic to certain plastics should not wear contact or lenses frames or lenses ocularsciencecontactlens manufactured from that type of plastic. People allergic to nickel should not wear Flexon frames. People at risk of being in accidents that might shatter glass lenses should wear plastic lenses, preferably polycarbonate. Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances ocularsciencecontactlens (usually about 28 in). Conventional bifocals and trifocals have visible lines between the areas of different correction; however, lenses where the correction ocularsciencecontactlens gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses are sometimes called progressives or no-line bifocals.To be suitable for lenses lenses, a material must be transparent, without bubbles, and 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. ocularsciencecontactlens The advantage of glass is that it is scratch resistant and provides the clearest possible vision. They can be worn up to a week. Daily wear lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. lenses are easier to get used to than rigid lenses, but are more prone to tears and do not last as long. Extended-wear lenses are highly permeable to oxygen, are flexible by virtue of their ocularsciencecontactlens ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems. There is more of a risk of infection with extended-wear lenses than with daily-wear ocularsciencecontactlens lenses. Extended-wear disposable ocularsciencecontactlens lenses ocularsciencecontactlens are lenses worn continually for up to six days and then discarded, with no need for cleaning.
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