When you are sitting behind bulletproof glass or any form of high-level security, you may be used to thinking that it is either this or that – but not both. Surely if you are behind bulletproof glass then bullets cannot go through it either way. Well, the truth of the matter is that people do construct bulletproof glass that will stop a bullet one way but will let bullets fly straight through the other way.
The physics of bulletproof glass has been long contended and researched, after all, they are an important part of modern society. From monarchs to presidents to the pope, everyone with a very high ranking profile in our modern world at some point sits behind bulletproof glass. You may think they are overly paranoid, but the risks of being shot by a lone sniper are just too great. Especially with how advanced modern sniper rifles have become, allowing high precision at long range.
It is, of course, natural to think that bulletproof glass is simply a substance that is strong enough to block bullets. And when we do think of it that way, it is pretty hard to see how it could let bullets fly through the other way. The secret is how the bulletproof glass is structured.
The engineering secret around the material is that it is not actually the material being used that blocks the bullets, it is how the material is put together. Traditionally bulletproof glass is made out of polycarbonate thermoplastic which is a special material which is known to be a rock hard substance and this material is layered between regular glass. The ultra-thick glass will, therefore, stop any motion of a bullet going in any direction, in fact, it will pretty much stop anything.
Here lies the difference with one-way bulletproof glass, it does have a hard substance also but only on the outside of the glass. On the inside you find a soft and flexible layer, when a bullet hits the hard exterior it will slow down so fast that the bullet actually flattens itself. Yet when you do the same from the inside, you will find that the soft layer will move with the bullet.
The movement will ultimately cause the hard layer to crack, allowing the bullet to fly through without any issues. That is some pretty amazing engineering.