Bullet proof glases contain polycarbonate layer sandwithed between two glass layers. How polycarbonate layers prevent bullets? Does it totally prevent piercing of glass? Is it directional i.e is from one side only?

yes, it is the polycarbonate that stops the bullet.

so the glass, on the side where it got shot, will break, but the opposite side wont.

 

and, to answer the other part, it does work 2 ways, however, there is a 1 sided glass (alloowing people to shoot through, but not be shot at).