Like any other material, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so it can bend rather than shatter. The refractive indices of the two glasses used in the bulletproof layer must be nearly identical to keep the glass transparent and to obtain a clear, undistorted view through the glass. Bulletproof glass is available in thicknesses from 3⁄4 to 3+1⁄2 inches (19 to 89 mm).
Bulletproof glass is used in the windows of buildings that require this security, such as jewelry stores and embassies, as well as in the windows of military and private vehicles.