In Young’s double slit experiment, intensity at the centre of screen is I0 . Find the minimum distance from centre of screen, when intensity is  I04

I=I0cos2θ2 ⇒I04=I0cos2θ2  ⇒cosθ2=12   ∴θ2 = π3        or         θ=2π3  Now,  ∆x= λ2πθ ⇒∆x= λ2π×2π3=λ3 ⇒ydD=λ3 ∴y=Dλ3d alternate solution: I=4I0cos2πxλ  where x is the path difference .now the distance of the required point is Y= xDd