
At maximum angle θ ray at point B goes in gazing emergence, at all less values of θ, TIR occurs. At point B
34×sinθ′′=1×sin90∘
θ′′=sin−1(43)
θ′=(2π−θ′′)
At point A
1×sinθ=34×sinθ′
sinθ=34×sin(2π−θ′′)
sinθ=34cos[cos−147]

sinθ=34×47
θ=sin−1(37)