The condition for maxima in diffraction is,
bsinθ=(2n+1)2λ.
For the first maxima n=1. Therefore,
bsinθ=23λ⇒sinθ=2b3λ=Dyy=2b3λD
Now the difference between the positions of the maxima will be,
y1−y2=2b3D(λ1−λ2)⇒y1−y2=2×5×10−43×2×(655−650)×10−9=3×10−5m