What is the strongest clear or see-through substance known to man and how thick must it be to be considered bulletproof?

Answer 1

There is no one definition of strongest, and NOTHING is "bullet proof"- materials are "bullet resistant". Bullet resistance is defined in regard to specific weapons and ammunition, such as a 9mm handgun, 30-06 rifle firing M2 ball ammo, etc. The "strongest" material I have worked with was lexan polycarbonate, as glazing for the artillery fire adjustment bunker at the Ft. Benning GA range complex. THAT material was 12 layers, each 1.5 inches thick, and intended to protect the occupants of the bunker from repeated hits from 105mm howitzers firing high explosive shells. In some uses, multiple layers of glass are bonded with a high strength plastic.

Answer 2

The strongest see-through substance known to man is synthetic sapphire, also known as transparent aluminum. To be considered bulletproof, sapphire would need to be a minimum of 5 millimeters thick to protect against most handgun rounds, while thicker variations would be needed to stop higher caliber bullets.