lensesesover 140 million people in the united states d wear lenseses. people whose eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. for people who y are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina. lenses work by changing the direction of light so that images come into o focus on the retina. the greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of l 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 (multifocal). contact and lenses one type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects for distant objects (about 20 ft from the eyes).

lensesesover 140 million people in the united states d wear lenseses. people whose eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. for people who y are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina. lenses work by changing the direction of light so that images come into o focus on the retina. the greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of l 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 (multifocal). contact and lenses one type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects for distant objects (about 20 ft from the eyes).

they can be sfx and theatrical 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 d highly permeable to oxygen, are flexible by virtue of their 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 y a risk o of infection with extended-wear lenses than with daily-wear lenses. extended-wear disposable lenses are lenses worn continually for up to six days and then discarded, with no need for cleaning.