From the appearance, a piece of bullet-proof glass and a piece of ordinary glass seem to have no difference, but this is where their similarities end. A relatively ordinary piece of glass can be shattered by just one bullet. However, depending on the thickness of the glass and the type of weapon used, bullet-proof glass can withstand attacks from one to multiple bullets. So, what gives bullet-proof glass the ability to resist bullets?

Bullet-resistant glass produced by different manufacturers varies. However, basically, it involves sandwiching a layer of polycarbonate material between ordinary glass layers, a process known as lamination. During this process, a substance is formed that resembles ordinary glass but is thicker than ordinary glass. Polycarbonate is a rigid transparent plastic - people usually refer to it by its brand name. The thickness of bullet-resistant glass ranges from 7mm to 75mm. A bullet fired at bullet-resistant glass will break through the outer layer of glass, but the polycarbonate layer can absorb the energy of the bullet, thus preventing it from penetrating the inner layer of glass.

The bulletproof capability of bulletproof glass depends on the thickness of the glass. The impact force of a rifle bullet on glass is much greater than that of a handgun bullet, so bulletproof glass designed to withstand rifle bullets is much thicker than that designed to withstand only handgun bullets.

There is also a type of one-way bullet-proof glass. One side of it can defend against bullets, but it does not hinder bullets from passing through the other side. This allows people under attack to retaliate. This type of bullet-proof glass is made by laminating a layer of brittle material and a layer of ductile material.

One can envision a car equipped with such unidirectional bullet-proof glass. If someone outside the car shoots at the window, the bullet will first hit the relatively brittle material layer. The brittle material near the impact point will shatter, absorbing some of the energy over a large area. The tough material will then absorb the remaining energy of the bullet, thus deflecting it. A bullet fired from inside the same car can easily penetrate the glass. Because the energy of the bullet hits a small area, the tough material rebounds outward. This also causes the brittle material to shatter outward, allowing the bullet to penetrate the tough material and hit the target.