CUET · PHYSICS · PYQ PAPER 2025

In Young's double slit experiment, two slits are 2 mm apart and the screen is 2 m away from the slits. A light source having two wavelengths 4000 \( \text{\AA} \) and 6000 \( \text{\AA} \) is used.
The minimum distance from the centre of the screen where the bright fringes produced by both wavelengths coincide is :

A \( 1.2 \times 10^{-4} \text{ m} \)
B \( 1.2 \times 10^{-3} \text{ m} \)
C \( 1.8 \times 10^{-4} \text{ m} \)
D \( 2.4 \times 10^{-3} \text{ m} \)

Verified Solution

Answer & Solution

Correct Answer

(B) \( 1.2 \times 10^{-3} \text{ m} \)

Step-by-step Solution

Detailed explanation

\( y = \frac{n \lambda D}{d} \) \( n_1 \lambda_1 = n_2 \lambda_2 \) \( n_1 (4000 \text{ Å}) = n_2 (6000 \text{ Å}) \Rightarrow 2n_1 = 3n_2 \) \( n_1 = 3, n_2 = 2 \) \( y = \frac{3 \times 4000 \times 10^{-10} \text{ m} \times 2 \text{ m}}{2 \times 10^{-3} \text{ m}} \)…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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