Two coherent sources S1 and S2 are separated by a distance four times the wavelength l of the source. The sources lie along y axis whereas a detector moves along + x axis. Leaving the origin and far off points the number of points where maxima are observed is

From  ΔS1S2D,(S1D)2=(S1S2)2+(S2D)2(S1P+PD)2=(S1S2)2+(S2D)2Here S1P  is the path difference =nλ  for maximum intensity. ∴(nλ+xn)2=(4λ)2+(xn)2or  xn=16λ2−n2λ22nλThen  x1=16λ2−λ22λ=7.5 λx2=16λ2−4λ24λ=3λx3=16λ2−9λ26λ=76λx4=0.∴Number of points for maxima becomes 3.