Define the term 'critical angle'. A small bulb is placed at the bottom of a tank containing a liquid of refractive index μ at a depth H. It is…

Critical angle: The angle of incidence corresponding to the angle of refraction $\displaystyle 90^{\circ}$ for a given pair of media when light goes from denser to rarer medium. \[\begin{gathered} \operatorname{Sin} i_{c}=\frac{1}{\mu}=\frac{r}{\sqrt{r^{2}+H^{2}}} \\ \mu^{2} r^{2}=r^{2}+H^{2} \\ \left(\mu^{2}-1\right) r^{2}=H^{2} \\ r=\frac{H}{\sqrt{\mu^{2}-1}} \end{gathered} \]