Thinner. Because of their ability to bend light more efficiently, high-index lenses for nearsightedness have thinner edges than lenses with the same prescription power that are made of conventional plastic material.
Lighter. Thinner edges require less lens material, which reduces the overall weight of the lenses. Lenses made of high-index plastic are lighter than the same lenses made in conventional plastic, so they're more comfortable to wear. High-index glass lenses also have thinner edges, but high-index glass is heavier than conventional glass, so there is not as much weight savings with glass as there is with plastic lenses. Lightweight lenses are even more of a benefit for farsighted prescriptions, which can make conventional lenses very heavy. And most high-index lenses also have an aspheric design, which gives them a slimmer, more attractive profile and reduces the magnified "bug-eye" look that conventional lenses cause in strong farsighted prescriptions.
High-index plastic lenses are available in a wide variety of refractive indices, typically ranging from 1.53 to 1.74. Lenses with an index of refraction of 1.70 or higher typically are at least 50 percent thinner than conventional plastic lenses.