Total internal reflection requires angle of incidence exceeding critical angle

Steps:

• Light incident at 90° to the surface means angle of incidence i = 90° (grazing incidence) at glass-air interfaces of both prisms.
• Critical angle c = 42° for glass to air.
• For TIR, i must exceed c; here 90° > 42°.
• Thus, TIR occurs at the interfaces in both P and Q.

Why A is correct:

• Both prisms have glass-air interfaces where i = 90° > 42° = c, satisfying Snell's law condition for TIR (sin i > sin c).

Why the others are wrong:

• B: Incorrect, as TIR also occurs in Q (same i > c).
• C: Incorrect, as TIR also occurs in P (same i > c).
• D: Incorrect, as i = 90° > c in both prisms.

Final answer: A