RRB ALP CBT I 11 Feb 2026 Shift 1 Paper

Concept:Refractive index compares the speed of light in air or vacuum with its speed in a given medium.Explanation:The refractive index is given by $n = \frac{c}{v}$, where $c$ is the speed of light in air or vacuum and $v$ is the speed of light in the medium.For water, $n = 1.33$, so $\frac{c}{v} = 1.33$.This means $c = 1.33 \times v$, i.e., light in air travels 1.33 times faster than light in water.Water is optically denser than air, so light slows down when it enters water.Option A is incorrect because light cannot travel faster in water than in air.Option C is incorrect because light moves faster in air, not slower.Option D is incorrect because the speed of light changes when it passes between media of different optical densities.The refractive index is a dimensionless quantity, and the speed of light in vacuum is about $3 \times 10^{8}\ \text{m/s}$.Frequency of light remains constant during refraction, while speed and wavelength change.Answer:Option B: The speed of light in air is 1.33 times the speed of light in water.