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JEE Main 2025 Jan 22 Shift 1, Physics Q4: Young’s Double Slit Experiment

JEE Main2025Jan 22, Shift 1Physics
Q.

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : If Young's double slit experiment is performed in an optically denser medium than air, then the consecutive fringes come closer.

Reason (R) : The speed of light reduces in an optically denser medium than air while its frequency does not change.

In the light of the above statements, choose the most appropriate answer from the options given below :

  1. A

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

  2. B

    (A) is false but (R) is true.

  3. C

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

  4. D

    (A) is true but (R) is false.

Solution

Fringe width: .

In a medium of refractive index , the speed of light is , frequency stays the same (set by the source), so the wavelength becomes , i.e. it shortens.

Smaller gives smaller , so consecutive fringes move closer. (A) is true and (R) is precisely the chain of reasoning that produces this conclusion, so (R) correctly explains (A).

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