Bullet-proof glass

Bulletproof glass is usually transparent material, such as polycarbonate thermoplastic (i.e. Lexan but also called Tuffak and Cyrolon), that provides the appearance and light-transmitting behavior of standard glass but offers varying degrees of protection from small arms fire.

Bulletproof glass usually consists of a polycarbonate layer sandwiched between layers of regular glass. A bullet pierces the exterior glass layer with ease, but the strong polycarbonate layer stops the bullets motion before it can pierce the inner layer of glass. Bulletproof glass is usually 7-75mm thick.

One-way bulletproof glass

Advances in bulletproof glass have lead to the invention of one-way bulletproof glass, such as used in some bank armored cars. This glass will resist incoming small arms fire striking the outside of the glass, but will allow those on the other side of the glass, such as guards firing from inside the armored car, to fire through the glass at the exterior threat.

One-way bulletproof glass is usually made up of two layers, a brittle layer on the outside and a flexible one on the inside. When a bullet is fired from the outside it hits the brittle layer first, shattering an area of it. This shattering absorbs some of the bullet's kinetic energy, and spreads it on a larger area. When the slowed bullet hits the flexible layer, it is stopped. However, when a bullet is fired from the inside, it hits the flexible layer first. The bullet penetrates the flexible layer because its energy is focused on a smaller area, the brittle layer then shatters outward due to the flexing of the inner layer and doesn't hinder the bullet's progress.