  In an earlier work (R. Bhandari, Phys. Lett. A 204 (1995) 188), it was shown
that, contrary to the property of achromaticity (independence of wavelength)
usually associated with topological phases, topological phases encountered in
polarization optics can, under certain conditions, show sharp changes and
reversals as a function of wavelength, a phenomenon originating in the
occurrence of phase singularities, earlier observed in interference
experiments. It was shown that a proposed flat lens capable of focussing light
with pure geometric phase shifts can switch from being convex to a concave lens
or vice versa under these conditions. In this paper conditions under which such
reversals can take place for small changes in wavelength are described. Some
general implications of phase singularities in the physics of interference are
pointed out.
