Q 12-10-017NEETJEE MainMedium
In Young's experiment, the $4^{\text{th}}$ dark fringe is at $7$ mm from the central maximum. The fringe width is
Answer: (B) $2$ mm
The $n^{\text{th}}$ dark fringe is at $\left(n - \dfrac{1}{2}\right)\beta$: $3.5\beta = 7$ mm, so $\beta = 2$ mm.
Solution by Sreeraj P, M.Sc Physics