Question

A light ray enters a prism through the side PR. PQR is a cross section of the prism. The emergent ray lies along the boundary OQ. ∠P = 90° and PQ = PR. The ray diagram has been given.

i. What is the critical angle of the prism?

[1 mark]

• A. 45°
• B. 30°
• C. 60°
• D. Cannot be determined

Answer 1

The correct option is A 45°

Given,
∠P = 90° and PQ = PR
△PQR is an isosceles triangle. Therefore, ∠R = ∠Q
∠R +∠Q = 90°
∠R =∠Q = 45°
∠OBR = 90°

Therefore, ∠BOR = 45°
∠BOC = 45°
Hence, the critical angle is 45°.