A glass prism is immeresed in water as shown in the figure. When a beam of light incident is incident normally on the face $\mathrm{ab}$, it is internally reflected at the face $\mathrm{ad}$. If the refractive index of glass is $1.5$ and that of water is $\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$, then