You have many options when you shop for lenses for your eyeglasses. Various eyeglass providers may carry different types of lenses, each with its own trademark name. For years, glass was the only lens material available, and glass still offers superior optics. It's the most scratch-resistant material, but the primary disadvantage of a glass lens is its weight, which is generally twice that of plastic lenses. Glass lenses may be heat- or chemically-treated to increase impact resistance. Since 1972, federal law has required that all lenses be shatter-resistant. Conventional hard-resin, or plastic, lenses are half the weight of glass lenses and can be tinted to almost any color and density. Hard-resin lenses are more easily scratched than glass, but they can have an optional scratch protection applied. They're more impact-resistant than glass lenses and don't require heat treating. Today's technology has resulted in lighter, thinner lenses-- called high-index lenses-- that bend light differently so that stronger corrections are thinner than when made using conventional materials. Glass lenses are also available with the advantages of high index, but they're considerably heavier. High-index plastic uses less material, so the lenses are often lighter in weight. These lenses can be coated to absorb all harmful UV (U-V) light and can be tinted to any shade or color. Another type of lens is the photochromic lens, which darkens as you go from indoors to outdoors to a moderate shade of gray. It's available in both glass and in lightweight, hard resin. Polycarbonate lenses are the most impact- resistant lenses available and are frequently the lens of choice for young people and active adults. Polycarbonate lenses are high index and are usually the lightest, most comfortable lenses. They absorb all harmful UV light and can be made with ultra-thin edges because of their unique strength.