UPSC NDA I 2015 General Ability Paper

Concept:Refraction changes the velocity of light because the optical density of the medium affects its speed.Explanation:When light passes from a rarer medium (e.g., air) to a denser medium (e.g., glass), its velocity decreases.This is because the refractive index $(n)$ of a denser medium is greater than 1, and $v = c/n$, where $c$ is the speed of light in vacuum.Snell’s law relates the angles and velocities: $\frac{\sin i}{\sin r} = \frac{v_2}{v_1}$.In going from rarer to denser, the ray bends towards the normal, so the angle of refraction $r$ is smaller than the angle of incidence $i$.Thus $\sin r < \sin i$, making the ratio $\frac{\sin i}{\sin r} > 1$. Hence $v_2 < v_1$ — the velocity decreases.For the reverse direction (denser to rarer), the ray bends away from the normal, and velocity increases.Therefore, the correct statement among the given options is that velocity decreases when moving from rarer to denser medium.Answer:Option C: Velocity decreases as the ray passes from a rarer to a denser medium.