We will introduce the Rayleigh plane-wave expansion exp(2πiu • r 12 ) = ∞ l=0 i l (2l + 1)P l (cos θ)j l (2πur 12 ), (6) where P l (x) is the lth order Legendre Polynomial, θ is the angle between r 12 and u, and j l (x) is the lth order spherical Bessel function. Note that from the orthogonality of the Legendre Polynomials (with P 0 (x) ≡ 1), by integrating both sides of (6) over the unit sphere all terms of the sum vanish except that for l = 0, leaving π 0 2π 0 exp(2πiu • r 12 ) sin(θ)dθdϕ = 4πj 0 (2πur 12 ). (7) We can use this result to evaluate the integral in (5) in spherical coordinates: