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MCQEasyJEE 2026Interference (Young's Experiment)

JEE Physics 2026 Question with Solution

Given below are two statements :

Statement I : In a Young's double slit experiment, the angular separation of fringes will increase as the screen is moved away from the plane of the slits

Statement II : In a Young's double slit experiment, the angular separation of fringes will increase when monochromatic source is replaced by another monochromatic source of higher wavelength

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

  • A

    Both Statement I and Statement II are true

  • B

    Both Statement I and Statement II are false

  • C

    Statement I is true but Statement II is false

  • D

    Statement I is false but Statement II is true

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Two statements about angular separation of fringes in Young's double slit experiment.

Find: Which statement is true.

Angular separation and linear fringe width are different quantities.

The linear fringe width is

β=λDd\beta = \frac{\lambda D}{d}

and the angular separation is

θ=βD=λd\theta = \frac{\beta}{D} = \frac{\lambda}{d}

For Statement I, angular separation

θ=λd\theta = \frac{\lambda}{d}

does not depend on the screen distance DD. Therefore, moving the screen away from the slits does not increase the angular separation. So Statement I is false.

For Statement II, from

θ=λd\theta = \frac{\lambda}{d}

we get that angular separation is directly proportional to wavelength. Hence, if the source is replaced by another monochromatic source of higher wavelength, the angular separation increases. So Statement II is true.

Therefore, the correct option is D.

Common mistakes

  • Confusing linear fringe width with angular separation. The quantity β=λDd\beta = \frac{\lambda D}{d} depends on screen distance, but angular separation θ=λd\theta = \frac{\lambda}{d} does not. First identify which quantity the statement refers to.

  • Assuming that increasing screen distance always increases every fringe-related measure. Only the linear spacing on the screen increases with DD; the angular separation remains unchanged because it is an intrinsic property of the slit arrangement.

  • Ignoring the dependence on wavelength. Since θ∝λ\theta \propto \lambda, a higher wavelength increases the angular separation. Do not treat monochromatic sources of different wavelengths as equivalent in YDSE.

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