High-index lenses are made out of a specific type of plastic, unsurprisingly called high-index plastic. This material and the way the lenses are formed causes light to be refracted in a unique way, and that refraction allows for a stronger prescription to be packaged in a much thinner lens than you’d be able to get using other materials.

People often get high-index lenses confused with polycarbonate lenses. This is due in part to the popularity of polycarbonate in the 1980s and beyond. Polycarbonate material is extremely strong and can hold up to rougher wear than some other materials, which is why it’s still recommended today for people who engage in rugged activity or work. It’s also still recommended for children due to the fact that it’s less likely to crack.