Critical angle in refraction at liquid-air boundary

Steps:

• Identify the incident ray from liquid (denser medium) to air (rarer medium).
• Note the refracted ray in air along the boundary, indicating 90° angle of refraction.
• Apply Snell's law: n_liquid * sin(i) = n_air * sin(r), where r = 90°, sin(90°) = 1.
• Thus, sin(i) = 1 / n_liquid, so i is the critical angle, and r = 90°.

Why D is correct:

• By definition, at the critical angle of incidence, the refracted ray in air grazes the boundary at 90° (Snell's law with sin(r) = 1).

Why the others are wrong:

• A: 0° would mean normal incidence with no bending, not matching boundary grazing.
• B: 40° is a typical refraction angle but not for grazing emergence.
• C: 50° exceeds critical angle for most liquids, causing total internal reflection instead.

Final answer: D