How can fringe width increase in Young's double slit experiment?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → by reducing the separation of the slits
In Young’s double slit experiment the fringe width is
\( \displaystyle \beta = \frac{\lambda D}{d} \)
where \( \lambda \) is the wavelength, D the distance between the slits and the screen, and d the separation of the two slits. The fringe width increases when \( \lambda \) or D increases, or when d is reduced.
Why the other options are incorrect
Option (1) by decreasing the distance between slit and screen — Incorrect. \( \beta\propto D \), so a smaller D gives a smaller fringe width.
Option (2) by decreasing the width of the slit — Incorrect. The width of each slit does not appear in the fringe width; it only changes the intensity and the diffraction envelope of the pattern.
Option (3) by increasing the width of the slit — Incorrect. For the same reason the slit width has no effect on \( \beta \).
NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “… We will have constructive interference resulting in a bright region when \( xd/D = n\lambda \). That is,
\( \displaystyle x = x_n = \frac{n\lambda D}{d}; \quad n = 0, \pm 1, \pm 2, \ldots \qquad (10.13) \)
… Thus dark and bright bands appear on the screen, as shown in Fig. 10.13. Such bands are called fringes. Equations (10.13) and (10.14) show that dark and bright fringes are equally spaced.”