Let intensity of light on screen due to each slit is I0.
So internity at centre of screen is 4I0(as cosϕ=1).
Intensity at distance y from centre,
I=I0+I0+2I0I0cosϕ
As we know, Imax=4I0. Therefore,
⇒2Imax=2I0=2I0+2I0cosϕ
⇒cosϕ=0
⇒ϕ=2π
Hence, kΔx=2π
⇒λ2π(dsinθ)=2π
⇒λ2d×Dy=21
⇒y=4dλD=4×10−35×10−7×1
=125×10−6
=125