Find the angle at which total internal refraction takes place when light is going from glass to air.

The angle at which this takes place is called the critical angle. 

We can apply Snell's law in this situation:  n₁ sin C1 = n₂ sin (90deg)

sin C = n₂ / n₁ 

If the second medium is air, then n₂ ~1, so sin C = 1/ n₁