Geometrical Optics

Consider light travelling from a medium $A$ to medium $B$ separated by a plane interface. If the light undergoes total internal reflection during its travel from medium $A$ to $B$ and the speed of light in media $A$ and $B$ are $2.4 \times 10^8 \mathrm{~m} / \mathrm{s}$ and $2.7 \times 10^8 \mathrm{~m} / \mathrm{s}$, respectively, then the value of critical angle is :