The most scratch-resistant material, the primary disadvantage of glass is its weight, generally twice that of hard resin. Glass lenses are heat or chemically treated to increase impact resistance. These are lenses that darken as you go from indoors to outdoors. They darken to a moderate shade of grey. Ask for a demonstration of photosensitive lenses.
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. Glass lenses are also available 'with the slimming advantages of High index but are considerably heavier. High index plastic uses less material so they are often lighter in weight. High index lenses absorb 0 harmful UV light and can be tinted to any shade or color. It is generally accepted that ultraviolet rays in' sunlight pose potential harm to the eyes.