What is T.I.R? - total internal reflection

When a light ray hits the boundary between 2 materials of different densities

Beyond a certain angle, the angle become critical so all the waves reflect back so it is known as T.I.R.

How to calculate the T.I.R:

Sin c = nr / ni

Sin critical angle = angle of reflection/angle of incidence

How to calculate refractive index:

η= sin i/ sin r

η= sine of the angle of incidence/ sine of the angle of refraction

Refractive index of:
air: 1.0002 water: 1.333
crown glass:1.517 dense flint glass: 1.655 diamond: 2.417