Explanation

(a) Critical angel $\theta_{C} = \sin^{- 1}\left( \frac{1}{\mu} \right)$
Wavelength increases in the sequence of VIBGYOR. According to Cauchy’s formula refractive index $(\mu)$ decreases as the wavelength increases. Hence, the refractive index will increase in the sequence of ROYGBIV. The critical angle $\theta_{C}$ will thus increase in the same order VIBGYOR. For green light the incidence angle is just equal to the critical angle. For yellow, orange and red the critical angle will be greater than the incidence angel. So these colours will emerge from the glass air interface.