Two transparent media $A$ and $B$ are separated by a plane boundary. The speed of light in medium $A$ is $2.0\times {10}^8 m s^{-1}$ and in medium $B$ is $2.5 \times {10}^8 m s^{-1}$. The critical angle for which a ray of light going from $A$ to $B$ is totally internally reflected is