The diagram is shown. The ray
PQ enters the prism from the side
AB, its angle of incidence is
0°. Thus it does not undergo any refraction. Its angle of refraction is also
0°. When this light ray reaches the base of the prism (BC), its angle of incidence (on the boundary
BC) is
60° which is more than its critical angle (
42°). Thus this incident light ray (
QR) does not enter into the next medium, instead it undergoes total internal reflection. When this reflected light ray (
RS) reaches the side
AC, its angle of incidence is
0°. Again, it does not undergo any refraction, which causes the angle of emergence to become
0°.