Bulletproof glass is a tool that is used to defend the everyday bank teller, all the way to the leader of the free world. It is a little hard to imagine that anything would be able to stop a heavy piece of metal traveling at 2,330 feet per second.
That is exactly what that a normal looking windshield does here.
So how does it do it?
First we can explain how a normal pane of glass is made and why it breaks the way it does.
Glass is formed from molten sand, which melts at a whopping temperature of 3090 degrees Fahrenheit. For perspective, wood will typically combust at 451 degrees Fahrenheit.
So when this molten sand cools, it doesn’t return to the small yellow sand granules that it was before. Instead, it changes into the transparent, brittle substance that makes up your windows.
What is the difference between normal glass and bullet resistant glass?
Bullet resistant glass contains a toughened version of your everyday glass. But that isn’t the secret.
It is layered with substance called polycarbonate to form a composite. While glass is typically tough but very brittle. Polycarbonate is excellent for absorbing impact.
As Chris Woodford put it, it is a lot like catching a baseball: if you slow down the impact by moving your hand backwards as you catch the ball, rather than taking it all at one time, your hand hurts less.
So when you alternate these two different substances, one tough and one absorptive, you slow down the bullet and distribute the impact throughout the glass.
Make enough layers of toughened glass and polycarbonate, you end up with a nearly impenetrable transparent wall that is capable of thwarting both bank robbers and would-be assassins alike.
And the research continues
Obviously there are tougher forms of glass and new substitutions for the polycarbonate are being formed every day.
So as long as people continue to develop more effective weapons, we will need to develop new ways of defending against them.