This solution assumes that the delta function source is located at the origin. If the source is located at an arbitrary source point, denoted by the vector r ′ and the field point is located at the point r , then we may represent the scalar Green's function (for arbitrary source location) as ψ ψ ( r | r ′ ) = e i k | r − − r ′ | 4 π π | r − − r ′ | .