Polycarbonate is a type of plastic (a thermoplastic polyester, if you really want to know). It's used in "bullet-proof" windows in addition to safety glasses because of its flexibility and strength.
Since this material it gives a little, you could hammer a nail through a polycarbonate lense without it shattering. (You guesssed it--don't try this at home while wearing your glasses!) Traditional heavier, hardened safety glass is only one tenth as impact-resistant as polycarbonate plastic. However, because the material gives a little, it needs to include a scratch-proof coating for durability.
One more advantage of polycarbonate lenses is that they naturally absorb 99% of UV rays so they protect your eyes not only from things like road debris, but harmful UV radiation.
Working Person's Store stocks a range of products utilizing this space-age plastic. Check out these WileyX Eyeglasses and DeWalt Eyewear, they are both made with polycarbonate lenses. It even protects some of our Pelican Flashlights and Streamlight Flashlights . Look around and save.