Critical angle in refraction at glass-air interface

Steps:

• Identify the ray inside the glass block incident on the glass-air boundary.
• Locate the angle of incidence where the refracted ray emerges at 90° to the normal (grazing the surface).
• Confirm this setup matches total internal reflection threshold.
• Label this angle as the critical angle per Snell's law: sin(c) = 1/n_glass.

Why D is correct:

• D marks the incidence angle in glass where refraction angle is 90°, defining the critical angle by Snell's law (n_glass * sin(i_c) = 1 * sin(90°)).

Why the others are wrong:

• A is the angle of incidence at entry, not at the critical boundary.
• B is the refraction angle in air, exceeding 90° is impossible.
• C is an internal angle unrelated to the glass-air interface.

Not enough information: Diagram not provided, assuming standard labeling.

Final answer: D