Physics - Geometrical Optics Question with Solution | TestHub

The critical angle ${\theta }_c$ is defined when light travels from an optically denser medium to an optically rarer medium. Thus, the options "air to glass" and "water to glass" are incorrect as light travels from rarer to denser medium in these cases.

The formula for the critical angle is given by $\sin {\theta }_c=\frac{{\mu }_R}{{\mu }_D}$, where ${\mu }_R$ is the refractive index of the rarer medium and ${\mu }_D$ is the refractive index of the denser medium.

For the critical angle to be minimum, the ratio $\frac{{\mu }_R}{{\mu }_D}$ must be minimum.

For glass to air: $\sin {\theta }_c=\frac{{\mu }_a}{{\mu }_g}=\frac{1}{1.5}=0.67$

For glass to water: $\sin {\theta }_c=\frac{{\mu }_w}{{\mu }_g}=\frac{1.33}{1.5}=0.89$

Since $0.67<0.89$, the critical angle is minimum when light travels from glass to air.

Answer: glass to air