Physics - Geometrical Optics Question with Solution | TestHub

The critical angle ${\theta }_c$ is given by $\sin {\theta }_c=\frac{1}{\mu }$.

For the critical angle to be minimum, the refractive index $\mu$ must be maximum.

According to Cauchy's relation, the refractive index depends on the wavelength $\lambda$ as $\mu =A+\frac{B}{{\lambda }^2}$. Thus, $\mu$ is inversely related to $\lambda$.

For $\mu$ to be maximum, the wavelength $\lambda$ must be minimum.

Among the given colors, blue light has the minimum wavelength.

Therefore, the critical angle is minimum for blue color.

Answer: blue colour