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Physics Question 41 – JEE-MAIN 2025

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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 :

Recall the definition of optical density and its relation to the speed of light and refractive index.

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Ninja Strategy•Independent Evaluation

Evaluate the truthfulness of Assertion (A) and Reason (R) independently before checking their relationship.

Step 1: Evaluate Assertion (A)✦ Active

Assertion (A) states that the refractive index of glass is higher than that of air. The refractive index of air is approximately 1.0003, and that of common glass is around 1.5. Since 1.5>1.0003, light travels slower in glass than in air, indicating that glass is optically denser than air. Thus, Assertion (A) is correct.

Step 2: Evaluate Reason (R)○ Expand

Reason (R) states that optical density of a medium is directly proportionate to its mass density. This is incorrect. Optical density is related to how much a medium slows down light, which is quantified by its refractive index. Mass density is the mass per unit volume. These two properties are not directly proportional. For instance, turpentine has a lower mass density than water but is optically denser (higher refractive index). Kerosene is also optically denser than water but has a lower mass density. Therefore, Reason (R) is incorrect.

Step 3: Conclude based on evaluation○ Expand

Since Assertion (A) is correct and Reason (R) is incorrect, the most appropriate option is that (A) is correct but (R) is not correct.

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