A rectangular glass slab $ABCD$, of refractive index ${n_1}$, is immersed in water of refractive index ${n_2}({n_1} > {n_2})$. A ray of light in incident at the surface $AB$ of the slab as shown. The maximum value of the angle of incidence ${\alpha _{\max }}$ such that the ray comes out only from the other surface $CD$ is given by <br>.