Total internal reflection (“TIR”) occurs, when the angle of incidence of a ray propagating from a higher indes medium to a lower index medium exceeds the critical angle.

Examples for critcal angles

1.3	50.3°
1.4	45.6°
1.5	41.8°
1.6	37.8°
1.7	36.0°
1.8	33.7°
1.9	31.8°
2.0	30.0°

for an interactive example, see refraction