Using \(\eta=\frac{1}{\sin C}\) or \(\sin C=\frac{1}{\eta}\) where, \(C=\) critical angle
$$
\begin{aligned}
\sin C &=\frac{1}{1.5} \\
\sin C &=0.667 \\ &=\sin ^{-1}(0.667)=41.8^{\circ}
\end{aligned}
$$