Total internal refraction occurs when a ray of light meets the following conditions.
1. light falls from an optically dense material to a less dense material.
2. The angle of incident in the denser media must be greater than the critical angle.
In the case above the fiber-optic is optically denser than water. Since the light is incident to fiber-optic-water interface, then one condition is fulfilled.
Checking the other condition we carry out the calculation below;
n1sinФ1=n2sinФ2
where c ⇒critical angle
1.53 sinc = 1.33 sin 90
sinc =0.86928
c = 60.37°
The angle of incident is (65°-70°) is greater than critical angle.
With certainty, there would be no refraction. Total internal reflection would occur.
