A parallel beam of light consisting of red and blue colours is incident normally on face AB of a right-angled prism ABC as shown in Fig. The refractive index of the material of the prism for red light is 1.39 and for blue light is 1.47. Light of which colour will emerge from face AC?

The critical angle i_c for red light ${\mathrm{i}}_{{\mathrm{C}}_{\mathrm{R}}}={\sin }^{-1}\left(\frac{1}{1.39}\right)\simeq 46°$

The critical angle i_c for blue light ${\mathrm{i}}_{{\mathrm{C}}_{\mathrm{B}}}={\sin }^{-1}\left(\frac{1}{1.47}\right)\simeq 43°$

The parallel rays fall normally on face AB go through undeviated. Their angle of incidence on face AC is i = 45°. Since 45° is less than i_c for red light but more than i_c for blue light, the blue light is total reflected at face AC and the red light emerges out of AC.