Abstract

The problem of anomalous phase propagation of a spherical wave at the focus has been discussed for the case of a diffracting aperture of arbitrary shape. The solution given by Kirchhoff's integral has been split up into an "incident light wave," which shows the distribution of light to be expected according to geometrical optics and a "diffracted wave," which may be thought of as due to scattering of the incident wave at the diffracting edge. A sudden change of phase by $\ensuremath{\pi}$ has been shown to occur already in the incident wave. Thus we may, in this sense, consider this phenomenon as a geometric optical one.The case of a circular diffracting aperture, the focus lying on the normal through its center, which has been treated usually, appears to be not very suitable for an experimental investigation of the discussed phenomenon. It is this particular shape of the diffracting edge, which produces diffraction phenomena of considerable light intensity along the optical axis. These, however, are not because of the existence of a focus, but only because of the particular shape of the diffracting aperture.