White light is used to illuminate the two slits in Young's double slit experiment, separation between the slits is b and the screen is at a distance d(>>b) from the slits. At a point on the screen, directly in front of the slits, certain wavelengths are missing. Some of these missing wavelengths are

both a and b

Path difference = $(d+\frac{b^2}{2d})-d=\frac{b^2}{2d}$

For missing wavelengths,

$(2n-1)\frac{λ}{2}=\frac{b^2}{2d}$

If n = 1, $\frac{λ}{2}=\frac{b^2}{d}$

If n = 2, $3×\frac{λ}{2}=\frac{b^2}{2d}$ or $λ=\frac{b^2}{3d}$