Define critical angle.

Critical angle:

1. The critical angle is the incidence angle at which the angle of refraction is ${90}^{\circ }$.
2. The light should flow from a denser optical medium towards a less dense optical medium.
3. The critical angle will be $48.6^{\circ }$ for water-air boundaries and $61.0^{\circ }$ for crown glass-water boundaries.

Mathematical representation:

$$\begin{gathered} \mathrm{The}\mathrm{critical}\mathrm{angle}=\mathrm{The}\mathrm{inverse}\mathrm{function}\mathrm{of}\mathrm{the}\mathrm{sine}\mathrm{of}\left(\frac{\mathrm{refraction}\mathrm{index}}{\mathrm{incident}\mathrm{index}}\right) \\ {\mathrm{\theta }}_{\mathrm{cric}}={\mathrm{Sin}}^{-1}\left(\frac{{\mathrm{n}}_{\mathrm{r}}}{{\mathrm{n}}_{\mathrm{i}}}\right) \end{gathered}$$

Refractive index:

A number determined by dividing the speed of light within a vacuum by the speed of light inside a denser medium is called the refractive index.