The angle of prism is A and one of its refracting surface is silvered. Light rays falling at an angle of incidence '2A' on the first surface return back through the same path after suffering reflection at the silvered surface. The refractive index of the material of the prism is
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Logic Tip: The phrase "returns back through the same path" or "retraces its path" is a classic physics code phrase meaning the ray hit the mirror surface exactly along the normal line. Instantly set the angle of incidence/reflection at that boundary to zero!