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JEE Physics 2025 Question with Solution

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

    (A) is not correct but (R) is correct

  • B

    Both (A) and (R) are correct and (R) is the correct explanation of (A)

  • C

    (A) is correct but (R) is not correct

  • D

    Both (A) and (R) are correct but (R) is not the correct explanation of (A)

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Assertion (A): Refractive index of glass is higher than that of air.

Find: Which option correctly states the truth value of Assertion (A) and Reason (R).

Assertion (A) is correct. Glass has refractive index approximately 1.51.5, whereas air has refractive index close to 11. Hence, glass has a higher refractive index than air.

Reason (R) is not correct. Optical density is not directly proportional to mass density in a strict way, and refractive index does not result from mass density alone. It depends more directly on how light interacts with the electrons and atomic structure of the material.

Therefore, Assertion (A) is correct but Reason (R) is not correct.

The correct option is C.

Detailed Analysis

Given: One assertion and one reason are provided.

Find: Whether each statement is correct, and whether the reason explains the assertion.

  1. Analyze Assertion (A): Glass has a higher refractive index than air. Common glass has refractive index around 1.51.5, while air has refractive index nearly 11. So Assertion (A) is correct.

  2. Analyze Reason (R): The statement claims that optical density is directly proportionate to mass density and therefore refractive index is proportionate to mass density. This is not generally true. Refractive index depends on the electronic structure, electron density, and atomic arrangement of the medium, not only on mass density.

  3. Check explanation: Since Reason (R) itself is incorrect, it cannot be the correct explanation of Assertion (A).

Therefore, the correct conclusion is that (A) is correct but (R) is not correct.

Hence, the correct option is C.

Common mistakes

  • Assuming that higher mass density always means higher refractive index. This is wrong because refractive index depends primarily on interaction of light with the material's electrons and structure. Compare optical behavior using material properties, not mass density alone.

  • Treating optical density and mass density as the same concept. This is incorrect because optical density refers to how strongly a medium slows light, whereas mass density refers to mass per unit volume. Do not use them interchangeably.

  • Marking option D because both glass and air are familiar materials with different densities. This is wrong because the reason itself is not universally correct. First test whether the reason statement is scientifically valid before checking explanation.

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