Critical angle at air glass interface is ${\theta }_1$ and that at water air interface is ${\theta }_2$. If $\mu g=\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\text{\hspace{0.17em}and\hspace{0.17em}}{\mu }_w=\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$, then critical angle at water-glass interface is