Young’s double slit experiment is carried out with two thin sheets of thickness 10.5 μm each and refractive indices μ₁ = 1.50 and μ₂ = 1.40 covering the slits S₁ and S₂ respectively. If white light of range 4000 $\stackrel{0}{\mathrm{A}}$ to 7800 $\stackrel{0}{\mathrm{A}}$ is used, then which wavelength will produce minima exactly at the centre O of the screen.