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 discounted 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 acuvue changing contact the direction lenses of light discounted so that images come into focus on the retina. The acuvue and contact greater the index lenses of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, discounted the greater the change in direction of light that passes through it, and the greater the correction.Lenses can acuvue be unifocal, with one correction for all contact 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); the remainder of the lens corrects for distant lenses objects (about 20 ft from the eyes).

Glass was the first material to be used for lenses lenses, and was used for several hundred years before plastic was introduced. The crown glass used for lenses lenses has an index of refraction of 1.52.Optical-quality acrylic was introduced for lenses use in the early 1940s, but because it was easily scratched, brittle, and discolored discounted rapidly, it did not supplant acuvue glass as contact the material of choice. Furthermore, it had a lenses relatively low index of refraction, so it wasn''t suitable for people with large refractive errors. A plastic called CR-39, introduced in the 1960s, was more suitable. Today, lenses wearers can also choose between polycarbonate, which discounted is the most impact-resistant material available for eyewear, and polyurethane, which has exceptional optical qualities and an index of refraction of up to 1.66, much higher acuvue than the conventional contact plastics used for lenses, and even higher than glass. Patients with high prescriptions should ask about high index material options for their lenses. Aspheric lenses are also lenses useful for high prescriptions.