Which of the following is not due to total internal reflection?

In the given figure,at the water-air interface,a light ray is incident at the critical angle. Find the value of $\mu_g$.

$A$ ray of light is incident on the hypotenuse of a right-angled prism after travelling parallel to the base inside the prism. If $\mu$ is the refractive index of the material of the prism, the maximum value of the base angle for which light is totally reflected from the hypotenuse is

$A$ beam containing red,green,and blue light is incident on a prism. Their refractive indices are $1.39, 1.44$,and $1.47$ respectively. Then,the prism:

Assertion : In optical fibre,the diameter of the core is kept small.
Reason : This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

$A$ cylindrical transparent rod of refractive index $\mu = \frac{2}{\sqrt{3}}$ is surrounded by air. $A$ light ray is incident at the midpoint of one end of the rod as shown in the figure. The angle of incidence $\theta$ for which the light ray undergoes total internal reflection at the side wall of the rod is: