We're used to thinking of things as either/or -– especially when those things are panes of bulletproof glass and we're sitting behind them. How do people make glass that can stop a bullet one way, but let bullets fly through them another way? We'll take a look at the physics of bulletproof glass.
It's natural to think of bulletproof "glass" as a substance that is strong enough to block bullets, even though it's clear. When we think of it that way, it's impossible to see how it could possibly block bullets going one way, but let them fly going the other way.
The secret to figuring out how one-way bulletproof glass works is to understand that the bullets aren't stopped due to the materials used, but due to the way the way those materials are structured. Ordinary bulletproof glass is polycarbonate thermoplastic (a very hard substance) layered between regular glass. The thick glass and the hard substance together stop the motion of the bullets. And they stop bullets from either direction.
One-way bulletproof glass has that hard substance as well, but it has it on the outside of the glass. There is a softer, flexible substance on the other side. When a bullet is fired at the outside of the glass, the hard layer slows the bullet down so suddenly that the bullet itself actually flattens a bit. This makes it both slower and wider when it gets to the soft layer. This layer drags at it, pulling it to a stop. When a bullet is fired from inside the glass, it's narrow and fast enough that it can fly through the flexible layer. The flexible layer moves enough that it puts stress on the brittle, hard outer layer and cracks it, allowing the bullet to more easily pierce that as well. Because of the structural difference, a bullet can fly out, but can't fly in. Let's hear it for engineering.