The intensity of the waves from sources S1 and S2 are I0. They interfere at D and the wavelength of the wave is λ=4 m. Resultant intensity at D will be
Path difference
Δx=s2D−s1D=5−4=1 m
∴ The corresponding phase difference will be
ϕ=2πλΔx=(2π4)(1)=π2
Using I=I1+I2+2√I1I2 cosϕ
I=I0+I0+2√I0I0 cosπ2=2I0