A ray of light is incident normally on one face of 30°– 60°– 90° prism of refractive index 5/3 immersed in water of refractive index 4/3 as shown in figure. 

The exit angle $\theta$₂ of the ray is ${\sin }^{-1}(5/4\sqrt{3})$

Total internal reflection at point P ceases if the refractive index of water is increased to 5/6 by dissolving some substance.

Total internal reflection at point P ceases if the refractive index of water is increased to $5/2\sqrt{3}$ by dissolving some substance 

The exit angle ${\mathrm{\theta }}_2$ of the ray is sin^–1(5/8)