Gaussian beam as a bundle of complex rays

It is observed that the function G(P)=eikr/r, where r is the distance from the observation point P to a fixed point having a complex location, represents the field of a Gaussian beam. This can be used to justify, without further computation, the application of formulas of ordinary optics to the transformation of beams through optical systems. It can also be used to solve very simply some problems of diffraction and scattering of Gaussian beams.