YDSE: Film, Medium & Two Wavelengths question from MHT CET 2026 (11 Apr, Shift 2)

Answer: (b)

1. In front of a slit $y = d/2$, so path difference $= \dfrac{yd}{D} = \dfrac{d^2}{2D}$.
2. Minimum: $\dfrac{d^2}{2D} = \dfrac{(2n-1)\lambda}{2} \Rightarrow \lambda = \dfrac{d^2}{(2n-1)D}$
3. So $\lambda \propto \dfrac{1}{D}, \dfrac{1}{3D}, \dfrac{1}{5D}, \ldots$, i.e. inversely proportional to $D, 3D, 5D, \ldots$
4. So the answer is (b).