WAVE OPTICS

I am giivng the algorithm....solve it urself

At any general point at height y above O
Δx=n₁dsinθ+n₂(S₂P-S₁P)=dsinθ+yD/d=d/2 +yD/d ---------(1)
8)
Find y for which Δx=0
and thus Δφ=0 for that y

9)
For interference at O

Δx=dsinθ

Find Δφ from this

then use formula for intensity at any point I_R=I₁+I₂+2√I₁I₂cosφ

ATQ::
I₁=I₂=I_o

Find I_R using all the info

10)
Δx=±mλ
Since needed very next maxima above put m=1
and calculate y using (1)

i got it.....but one doubt why u mutiplied dsin θ with n₁

becoz its in medium of refractive index n1

thanks [1]