eye disorders
Corrective Lenses
Eyeglasses
Eyeglass lenses can be made of either plastic or glass. Plastic lensesfor eyeglasses are lighter but are more likely to scratch. These lenses are more commonly used than glass because they are thinner and can also be coated with a substance that helps them resist scratches. Glass lensesfor eyeglasses are more durable and less likely to scratch than plastic but are more likely to break, potentially causing injury. Both glass and plastic lenses can be tinted or treated with a chemical that darkens them automatically when exposed to light. Lenses can also be coated to reduce the amount of potentially damaging ultraviolet light that reaches the eye. Newer plastic lenses (made from polycarbonate or a urethane-based monomer) completely block ultraviolet light, are highly resistant to breaking, and also can be coated with a scratch-resistant substance. Bifocals are eyeglasses that contain two lenses—an upper lens that corrects the view of distant objects and a lower lens that corrects the view of nearby objects, such as reading material. However, people also need to focus at middle distances, such as when viewing a computer screen. Trifocals are eyeglasses that meet this need because they contain a third lens for middle distance. Continuously variable lenses (progressive lenses or no-line bifocal lenses) also permit focusing at middle distances and have a cosmetic advantage in that there is no line or sharp division between the regions of the eyeglass lens. 