It is important once you have chosen your spectacle frame that a suitable lens for your prescription and frame is chosen.
Traditionally spectacle lenses were made in crown glass which was tough and durable. Glass is still available in single vision, bifocal and varifocal forms though in a reduced number of designs. Thinner glass lenses are now available which can reduce the edge (or centre) thickness by up to 40%, although they do not reduce the weight unfortunately. Largely for that reason most lenses today are made from plastics. In addition to your personal prescription, the weight and thickness of your lenses will be determined by the type of plastic used. Concerns about plastic lenses scratching more easily than glass are less valid these days due to the protective hard coatings which can be applied.
Whatever material is used to manufacture a spectacle lens, the higher its ‘refractive index’ the thinner the lens will be. Standard plastic lenses have a refractive index of 1.5, however thinner and lighter alternatives (offering optimum comfort and cosmetic appearance) are available in 1.6 to1.74 index. Plastic lenses can be UV coated, hard coated, anti reflection coated or tinted to suit your needs. They can also be polarised or photochromic if desired.