UPSC NDA I 2018 General Ability Solved Paper

Concept:The refractive index of a material with respect to air is the ratio of the speed of light in air to that in the material. Since air has a refractive index just above 1 and all other materials have higher indices, this ratio is always greater than 1.Explanation:• Refractive index of a medium is $n = \frac{c}{v}$, where $c$ is speed of light in vacuum and $v$ is speed in the medium.• In air, $v \approx c$, so $n_{\text{air}} \approx 1.0003$.• For any solid or liquid, $v$ is less than in air, hence $n_{\text{material}} > n_{\text{air}}$.• The refractive index of a material with respect to air is $\frac{n_{\text{material}}}{n_{\text{air}}}$. Since $n_{\text{material}} > n_{\text{air}}$, this ratio is always $>1$.• For example, water ($n=1.33$) gives a relative index of about 1.33, which is $>1$. Thus, the correct statement is that it is always greater than one.Answer:It is always greater than one.