Most clip-on sunglasses have polycarbonate lenses. They’re less expensive to produce than glass lenses and much less likely to shatter, an important consideration around the eyes. Additionally, they’re lightweight, which is especially important since they’ll be adding weight to your prescription glasses. Polycarbonate’s formulation makes it automatically resistant to ultraviolet rays. Polycarbonate is less scratch-resistant than glass, but it can be treated with a special coating to help protect it.