Refractive index of water with respect to air is 4/3. The critical angle for a water–air interface is approximately

1. For TIR going from water to air: sin θc = n_air / n_water = 1 / (4/3) = 3/4. 2. θc = sin⁻¹(0.75) ≈ 48.6°. 3. (B) is undefined (sin⁻¹ > 1); (C) and (D) ignore the n-ratio. _Source: syllabus Physics Part II, Ch 9 §9.4 + Table 9.1_