Bullet proof glasses are two layers of glasses with a layer of transparent plastic between them. The plastic layer is made of polycarbonate which is tough and transparent. The process of layering a polycarbonate material within two layers of glass is called lamination.
Bullet-resistant glass is between 7 millimeters and 75 millimeters in thickness. A bullet fired at a sheet of bullet-resistant glass will pierce the outside layer of the glass, but the layered polycarbonate-glassmaterial is able to absorb the bullet's energy and stop it before it exits the final layer.