Correct option: (B) 2 cos A
To retrace its path light rays should fall normally on the reflecting surface. So
r2 = 0 ⇒ r1 = A – r2 ⇒ r1 = A
Now applying snell rule between incident ray and refracted ray.
(A) sin (2A) = n sin (A) ⇒ 2 sin A cos A = n sin A
⇒ n = 2 cos A