In Young's double slit experiment, if the source of light changes from orange to blue then :

Solution

Physics sir · 4 steps

Fringe width goes as wavelength, and blue has a shorter wavelength than orange, so the fringes squeeze in.

1. Fringe width. The distance between two consecutive bright fringes is β = (λ D)/d. Here D is the slit-to-screen distance and d is the slit separation.

   β = (λ D)/d ⇒ β ∝ λ

2. Compare wavelengths. Orange light has λ ≈ 600 nm and blue light has λ ≈ 450 nm. With D and d unchanged, a smaller λ gives a smaller β.
3. Other options. At the centre the path difference is zero for every wavelength, so the central fringe stays bright. The minima stay at zero intensity as long as the two slits are equally bright, whatever the colour.
4. So. Switching from orange to blue decreases the distance between consecutive fringes. The answer is (b).

The idea to keep. In YDSE, fringe width β = λ D/d. Change the colour and only λ changes: towards violet the fringes crowd together, towards red they spread out.

Answer from the NTA final answer key. Solution by PrepWiser.