lensesesover 140 million people in the united states 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 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 focus on the retina. the greater the index of refraction of the lens material and theatrical the greater the difference in the curvature between 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 (multifocal). 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); homony the remainder of the lens corrects for distant objects (about 20 ft from the eyes).

lensesesover 140 million people in the united states 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 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 focus on the retina. the greater the index of refraction of the lens material and theatrical the greater the difference in the curvature between 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 (multifocal). 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); homony the remainder of the lens corrects for distant objects (about 20 ft from the eyes).