Answer:
Refractive index for diamond wrt air = 2.42. Refractive index of water wrt air = 1.33. Refractive index of diamond wrt water = Refractive index of diamond wrt air*Refractive index of air wrt water = 2.42*(1/1.33) = 1.82. Critical angle for light passing from diamond into water = arcsin (1/1.82) = 33.33 degrees.