Superposition, Coherence & Intensity question from JEE Main 2026 (24 Jan, Shift 2)

Answer: (b)

1. In front of one slit, $y = \dfrac{d}{2} = 1\ \text{mm}$ from the centre.
2. Path difference $\Delta x = \dfrac{yd}{D} = \dfrac{(10^{-3})(2\times10^{-3})}{10} = 2\times10^{-7}\ \text{m} = \dfrac{\lambda}{3}$ (as $\lambda = 6\times10^{-7}$ m).
3. $\phi = \dfrac{2\pi}{3}$, so $I = 4I_o\cos^2 60^\circ = I_o$.
4. So the answer is (b) $I_o$.