  We show that a time-reversed formulation of Huygens-Kirchhoff diffraction can
be used to deduce the transverse and longitudinal fields of a Gaussian laser
beam, starting from a simple assumption of a Gaussian beam profile in the far
field. An attempt to apply this technique to the far fields of a Hertzian
dipole shows how the laws of diffraction do not permit a wave to be focused to
a volume smaller than a cubic wavelength in a charge-free region.
