(c) Resultant intensity of interferring wave at ‘P’ Ip=I1+I2+2√I1I2cosϕ For ϕ=
π
2
,Ip=I+9I=10I Again resultant intensity at 'Q' IQ=I1+I2 + 2√I1I2cosϕ For ϕ = π, IQ = I+9I+(−2√9(I)2) = 10I−6I=4I ∴ Difference between the resultant intensity ΔI = IP - IQ = 10I−6I=4I