Superposition, Coherence & Intensity question from JEE Main 2026 (6 Apr, Shift 2)

Answer: (c)

1. Each slit gives $I_0$; resultant $I = 4I_0\cos^2\frac{\phi}{2}$.
2. $4I_0\cos^2\frac{\phi}{2} = I_0 \Rightarrow \cos\frac{\phi}{2} = \frac{1}{2} \Rightarrow \phi = \frac{2\pi}{3}$
3. $\Delta x = \frac{\lambda}{2\pi}\cdot\frac{2\pi}{3} = \frac{\lambda}{3} = \frac{6000}{3} = 2000\ \text{Å}$
4. So the answer is (c) 2000.