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
Concept. The $n$th dark fringe is at $\left(n - \dfrac12\right)\beta$.
Step 1. Solve. $3.5\beta = 7$ mm $\Rightarrow \beta = 2$ mm.
Answer. $2$ mm (Option B).
Solution by Sreeraj P, M.Sc Physics