1. What will the NA of the fibre be if it's immersed in water? Basically we know the NA = 0.30 and refractive index is 1.50 for the fibre's core. 2. Snell's law 3. My theory is that NA is the sine of the critical angle. n1 = 1.5, n2 = 1.333 NA = sin (theta_crit) = sin (arcsin (n2/n1)) = n2/n1 = 0.89 But our teacher said it was not correct but on the right track. Is there a possibility that the NA doesn't even change when the fibre is immersed in water?