Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : In Young's double slit experiment, the fringes produced by red light are closer as compared to those produced by blue light. Reason (R) : The fringe width is directly proportional to the wavelength of light. In the light of the above statements, choose the correct answer from the options given below :

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

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

Wave Optics PYQ — JEE Main 2025 · 22 Jan · Shift 2 · Q67 with Solution

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Correct answer: C

Use the formula for fringe width in YDSE

In Young’s double slit experiment, the fringe width is $\beta = \frac{\lambda D}{d}$ where:
- $\lambda$ = wavelength of light
- $D$ = distance of screen from slits
- $d$ = separation between slits
Analyse the Reason (R)

From the formula, $\beta \propto \lambda$ So the fringe width is directly proportional to wavelength.

Therefore, Reason (R) is true.
Analyse the Assertion (A)

Red light has a larger wavelength than blue light: $\lambda_{\text{red}} > \lambda_{\text{blue}}$

Since $\beta \propto \lambda,$ we get $\beta_{\text{red}} > \beta_{\text{blue}}$

This means fringes due to red light are farther apart, not closer.

Therefore, Assertion (A) is false.
Choose the correct option
- Assertion (A): False
- Reason (R): True
Hence, the correct option is: $\boxed{\text{C}}$
Comparison with stored answer

Stored correct answer: $\text{C}$

Our derived answer matches the stored answer.