Two spherical waves of equal intensity I0, originating at the points (- a, 0, 0) and (a, 0, 0), interfere in the plane z=d. Use the paraboloidal approximation for the spherical waves to show that the intensity at the plane z=d is  

Where the angle subtended by the centers of the two waves at the observation place is θ ≈ 2a/d. The intensity pattern is periodic with period λ/θ.