The path difference between two secondary wavelets is given by nλ = asinθ. Since, θ is very small sinθ = θ. So, for the first order diffraction n = 1, the angle is λ/a. Now we know that θ must be very small θ = 0 (nearly) because of which the diffraction pattern is minimum. Now for interference case, for two interfering waves of intensity I1 and I2 we must have two slits separated by a distance. We have the resultant intensity, I=I1+I2+2I1I2cosθ Since, θ = 0 (nearly) corresponding to angle λ/a so cosθ = 1 (nearly) So, I=I1+I2+2I1I2cosθ⇒I=I1+I2+2I1I2cos(0)⇒I=I1+I2+2I1I2 We see the resultant intensity is sum of the two intensities, so there is a maxima corresponding to the angle λ/a. This is why, at the same angle of λa, we get a maximum for two narrow slits separated by a distance "a".