transparent armour
A window you can see clearly through that will also stop a bullet sounds like a contradiction, and for a long time the only answer was thick, heavy stacks of glass. Transparent armour is the ceramic solution. The trick is to put a very hard, clear ceramic on the strike face: it is so hard that it shatters or blunts the incoming projectile, wrecking the tip that does the penetrating, while tough backing layers of glass and plastic catch the broken fragments and soak up the energy. You get to see through it because the ceramic is genuinely transparent, not merely a peephole.
For a polycrystalline ceramic to be transparent it must be almost perfectly free of pores (a pore scatters light like a bubble in ice) and it must be optically isotropic — a cubic crystal that bends light the same way in every direction, so grain boundaries do not scatter. Two cubic ceramics fit the bill: magnesium aluminate spinel (MgAl2O4) and ALON, an aluminium oxynitride (a solid solution in the aluminium-oxygen-nitrogen system). Both are far harder than glass — spinel around 14 to 16 GPa, ALON around 18, single-crystal sapphire above 20 — so an armour package can be lighter and thinner than an all-glass one of the same protection. In use they are laminated as a hard ceramic front bonded to glass and a polycarbonate backing.
Transparent armour matters for vehicle and aircraft windows, blast shields, and clear domes and windows for infrared sensors that must survive rain, sand and impact. Be honest about how it works and what it costs. The ceramic front does not 'stop' the bullet by itself; it defeats and erodes the projectile, and the whole layered system stops it. Single-crystal sapphire is even harder but is anisotropic and expensive; spinel and ALON win by being cubic and manufacturable as large plates. And the manufacturing is the hard part — growing large, flawless, pore-free transparent panels by hot pressing or sintering plus hot isostatic pressing is slow and costly, and the finished ceramic is still brittle.
An ALON window a little over an inch thick can defeat a round that would need nearly twice the thickness and weight of ordinary laminated bullet-resistant glass — the hard ceramic front breaks up the bullet before the glass-and-plastic backing has to catch it.
A hard cubic ceramic breaks the projectile; the tough backing catches the pieces — armour you can see through.
Transparency demands a cubic crystal and near-zero porosity. A birefringent (non-cubic) ceramic like ordinary alumina scatters light at every grain boundary and turns milky, no matter how pure it is.