Where can the phenomenon of total internal reflection be observed?

$A$ light ray travels from a medium with refractive index $n_1$ to another medium of refractive index $n_2$. If $n_1=2$ and $n_2=\sqrt{3}$, then find the critical angle. (in $^{\circ}$)

$A$ diver at a depth of $12 \ m$ in water $\left(\mu = \frac{4}{3}\right)$ sees the sky in a cone of semi-vertical angle:

$A$ ray is incident on a prism $ABC$ and travels as shown in the figure. The minimum refractive index of the material of the prism should be .......

Explain the use of optical fibre in the medical field.

The reason for the shining of an air bubble in water is