YDSE: Fringe Width & Fringe Positions question from MHT CET 2026 (13 Apr, Shift 2)

Answer: (d)

1. $Q$ is the 2nd bright fringe above $O$; $P$ is 11 bright fringes from $Q$ on the other side, so $P$ is the $11 - 2 = 9$th bright fringe below $O$.
2. $S_1B$ is the path difference $S_1P - S_2P$ at $P$, which equals $9\lambda$.
3. $S_1B = 9 \times 6000\times10^{-10} = 5.4\times10^{-6}\text{ m}$
4. So the answer is (d) $5.4\times10^{-6}$ m.