How does bullet proof glass work?
There is no bullet proof glass there is bullet resistant glass which is a form of toughened glass made of several layer that absorb the energy of the bullet.
Most bullet proof glass (that is clear) is more accurately called “bullet resistant layers of clear plastic”. Different types of plastic have different properties, such as one might squishy (can absorb movement without breaking) while another might be tougher (can stop movement, but might break easier). Occasionally different types of actual glass can be used as well, but rarely without another layer of plastic involved.
Bullet resistance is achieved by combining these properties, such as squishing enough to absorb a bullet’s movement while being tough enough to stop squishing before the bullet passes through it. The designers will take thicker and thicker layers of plastic, combine them in different orders, use different glues to keep them together, and sometimes just use some other materials (like a metal mesh or classic glass) to complement the plastic.
And when they actually resist the bullet, there is usually a scratch or crack the weakens it, so they will often be replaced after a single blocked bullet (logistics allowing). Just like a Kevlar vest needs to be replaced when it blocks a bullet, but with the added fact that its transparency is messed up as well.
A bit like an ogre – layers
Bullet proof glass is generally a bunch of layers of hard glass and squishy rubber/plastic.
The bullet hits an outer layer of glass and gives up some energy deforming and shattering that layer of glass, that glass then spreads the energy through the squishy layer so it doesn’t crack the next layer of glass and the process repeats with the cracking of the glass absorbing a fair amount of energy
Bullet proof glass is generally quite thick which gives it more space to burn off the energy of the incoming bullet. The more powerful a bullet it needs to stop the thicker it needs to be.