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, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft citation needed.
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 to 3 1⁄2 inches (19 to 89 mm). Bulletproof glass is used in windows of buildings that require security, such as jewelry stores and embassies, as well as military and private vehicles.