A ray of light PQ is incident normally on the face AB of an equilateral prism. The ray gets totally reflected from the surface AC.

From the below diagram,

(a) As, the incident ray and emergent ray makes an angle of 60° with each other.

So, angle of deviation at AC is 60°.

(b) Here, emergent ray is falling normally on face BC.

So, angle of incidence at BC is 0°.