A glass slab of refractive index 1.5 is cut in the shape of a parallelepiped ABCD as shown in the figure. Surface BC is polished to form a perfect reflector. Light is incident at surface AB from inside at an angle \pi/3. Which one of the following is the correct angle ABC (\theta) such that the reflected light retraces its path?

  1. A. \pi/6
  2. B. \pi/3 ✓
  3. C. \pi/4
  4. D. 5\pi/12

Correct Answer: B. \pi/3

Explanation

For the ray to retrace its path, it must strike the polished surface BC normally (90 degrees). After total internal reflection at AB (since 60 degrees > critical angle), the geometry of the triangle dictates that \theta + 30^\circ = 90^\circ, giving \theta = 60^\circ or \pi/3.

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