In YDSE experiment, the separation between the slits is equal to 3λ where λ is the wavelength of the light incident on the plane of the slits. A thin film of thickness 2λ and refractive index 2 has been placed in front of the upper slit. Location of central maxima on the screen is
2D3
ydD=Δx=(μ−1)t
y=Dd(μ−1)t
=D3λ(2−1)2λ=23D