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

For light to retrace its path after reflection from surface BC, the ray inside the slab must strike BC normally (perpendicular to BC). The light enters surface AB making an angle π/3 with the normal to AB (i.e., π/3 from normal of AB). For the ray to hit BC at 90° (along the normal of BC), the geometry requires the angle between AB and BC (which is angle ABC = θ) plus the propagation direction relate via θ = π/2 - π/3 = π/6. Hence θ = π/6 makes the ray inside the slab strike BC normally, ensuring it retraces its path back through AB.