CBSE Class 12 · 2022 · Geometrical OpticsMultiple Choice

Since $\sin {\theta }_c=\frac{1}{\mu }$, the critical angle is \emph{largest} for the medium of \emph{smallest} refractive index. Among the options water has the lowest $\mu$ ($\approx 1.33$), so it has the maximum critical angle. \textbf{Note:} the printed answer key marks Diamond, which is incorrect — diamond ($\mu \approx 2.42$) has the \emph{smallest} critical angle; the physically correct answer is Water.