Critical angle: refracted ray grazes surface at 90° to normal

Steps:

• Light enters semi-circular glass block through flat edge, refracts at curved glass-air interface.
• Critical angle occurs when incidence angle in glass makes refracted ray in air perpendicular to normal (90° angle).
• Diagrams depict ray paths; identify where emerging ray runs parallel to curved surface.
• Compare: only option C shows refracted ray at exactly 90° to normal at interface.

Why C is correct:

• Refracted ray emerges along surface, perpendicular to normal, matching critical angle definition for glass-air boundary.

Why the others are wrong:

• A: Refracted ray bends into air at <90° to normal, so incidence angle < critical.
• B: Ray reflects internally without emerging, indicating incidence > critical.
• D: No refraction shown; total internal reflection occurs, incidence > critical.

Final answer: C