Bulletproof glass (also known as ballistic glass, transparent armor or bullet-resistant glass) is a type of strong but optically transparent material that is particularly resistant to being penetrated when struck. Like any material, however, they are 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 flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from 3/4 in. to 3.5 in. (19mm to 76mm).