Thinner, Lighter Lenses For Your Eyeglasses
"High-index eyeglass lenses are the right choice if you want thinner, lighter lenses and eyeglasses that are as attractive and comfortable as possible. Thinner, lighter high-index lenses are especially recommended if you have a strong eyeglass prescription for nearsightedness, farsightedness or astigmatism. But high-index lenses can make your eyeglasses noticeably slimmer, lighter and more attractive regardless of how strong your prescription is. Most eyeglass wearers are nearsighted, which requires corrective lenses that are thin in the center but thicker at the edge of the lens. The stronger the prescription, the thicker the edges (see lens drawings below). Most of today's fashionable frames are made of plastic or metal with rims thinner than the lens itself. Also, popular rimless mountings mean that the edges of the lenses are completely exposed. In either case, the lens edges are highly visible, and thicker edges can detract from the appearance of your eyewear. Eyeglass lenses correct refractive errors by bending (refracting) light as it passes through the lens. The amount of light-bending ability (lens power) that's needed to provide good vision is indicated on the eyeglass prescription provided by your eye doctor."
Source - All About Vision