Retroreflection requires aligning mirror normal with incident ray

Steps:

• Initial angle of incidence is 45° to the normal.
• For light to reflect back along its path, angle of incidence must be 0°, so normal aligns with incident ray.
• Initial normal is 45° from incident ray direction.
• Rotate mirror 45° clockwise to align normal with incident ray.

Why C is correct:

• Law of reflection states angle of incidence equals angle of reflection; for retroreflection, both must be 0°, requiring 45° rotation from initial setup.

Why the others are wrong:

• A: 22.5° is half the needed angle, insufficient for alignment.
• B: 22.5° anticlockwise rotates in wrong direction.
• D: 45° anticlockwise rotates opposite to required direction.

Final answer: C