JEE Main 2025 · 7 April · evening · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R) .
Assertion (A): Refractive index of glass is higher than that of air.
Reason (R): Optical density of a medium is directly proportionate to its mass density which results in a proportionate refractive index.
In the light of the above statements, choose the most appropriate answer from the options given below:
- (a) Both (A) and (R) are correct and (R) is the correct explanation of (A)
- (b) (A) is correct but (R) is not correct
- (c) (A) is not correct but (R) is correct
- (d) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
Answer and solution
Answer (b) (A) is correct but (R) is not correct
Solution
Physics sir · 4 steps
Optically denser means light slows down more, not that the medium is heavier. Kerosene breaks R.
- Check the assertion. Refractive index is n = c/v, where v is the speed of light in the medium. Light slows down far more in glass (n ≈ 1.5) than in air (n ≈ 1.0003), so A is correct.
- What optical density means. A medium is optically denser when light travels more slowly in it, which gives a larger n. This depends on how the medium's atoms interact with light, not on its mass per unit volume.
Liquid Mass density Refractive index Water 1.00 g cm⁻³ 1.33 Kerosene ≈ 0.8 g cm⁻³ ≈ 1.44 Turpentine ≈ 0.87 g cm⁻³ ≈ 1.47 - Test the reason. Kerosene is lighter than water (it floats on water), yet its refractive index is higher. So refractive index is not proportional to mass density, and R is false.
- So. A is correct and R is not correct, which is option (b).
The idea to keep. Optically denser means light moves slower, so n is larger. Mass density is a different property: kerosene floats on water but bends light more.
Answer from the NTA final answer key. Solution by PrepWiser.