For a ray of light, the critical angle is minimum, when it travels from

The critical angle \(c\) for total internal reflection (TIR) when light travels from a medium of refractive index \(n_1\) to a medium of lower refractive index \(n_2\) is given by \(\sin c=\frac{n_2}{n_1}\) where \(n_1>n_2\). The smaller the ratio \(\frac{n_2}{n_1}\), the smaller \(\sin c\), and hence the smaller the critical angle \(c\). From the options, the valid scenarios for TIR are only those in which the first medium has a higher index than the second. That is: \(\begin{aligned} & \text { Glass to air }\left(n_{\text {glass }} \approx 1.5, n_{\text {air }} \approx 1.0\right) \\ & \sin c=\frac{1.0}{1.5}=\frac{2}{3} \quad \Longrightarrow \quad c \approx 42^{\circ} \end{aligned}\) Glass to water \(\left(n_{\text {glass }} \approx 1.5, n_{\text {water }} \approx 1.33\right)\) \(\sin c=\frac{1.33}{1.5} \approx 0.887 \quad \Longrightarrow \quad c \approx 62^{\circ}\) Clearly, the critical angle is smaller for glass-to-air than for glass-to-water. Hence, the critical angle is minimum when light travels from glass to air. Answer: (A) glass to air.