Refraction at the Critical Angle

Steps:

• Define critical angle: incidence angle in denser medium (glass) where refraction angle in rarer medium (air) is 90°.
• Apply Snell's law: n_glass * sin(c) = n_air * sin(90°), so sin(c) = 1/n_glass (≈1.5, c≈42°).
• At exactly c, refracted ray grazes surface parallel to interface; partial reflection occurs.
• Diagram must show incident ray at c, reflected ray, and refracted ray at 90° to normal.

Why D is correct:

• Depicts refracted ray along surface (90° to normal), matching Snell's law definition for critical angle.

Why the others are wrong:

• A: Shows total internal reflection (no refraction), which occurs only for incidence > c.
• B: Shows refraction at acute angle (<90°), valid for incidence < c.
• C: Shows straight transmission without bending, ignoring refraction at interface.

Final answer: D