(4) μ1 = μ4
Let i1 be angle of incidence of ray AB in medium of refractive index μ1 . Let, r1 be angle of refraction in medium 2. From Snell’s law
μ1 sin iI = μ2 sin r1 .....(1)
The angle of incidence, in medium μ2 is r1 . Let angle of refraction, in medium 3 be r2 . Then
μ2 sin iI = μ3 sin r2 .....(2)
The angle of incidence in μ3 is r2 and angle of emergence of ray CD in medium μ4 is i1 (because CD is parallel to AB). Therefore