  We investigate the formation of multiple images as the radio signals from
fast radio bursts (FRBs) pass through the plane of a plasma clump. The
exponential model for the plasma clump is adopted to analyze the properties of
the multiple images. By comparing with the classical dispersion relations, we
find that one image has exhibited specific inverse properties to others, such
as their delay times at high frequency is higher than that at low frequency,
owing to the lensing effects of the plasma clump. We demonstrate that these
inverse effects should be observable in some repeating FRBs. Our results
predict deviation in the estimated DM across multiple images, consistent with
the observations of FRB 121102 and FRB 180916.J0158+65. If other plasma lenses
have effects similar to an exponential lens, we find that they should also give
rise to the similar dispersion relation in the multiple images. For some
repeating FRBs, analysis of the differences in time delay and in DM between
multiple images at different frequencies can serve as a method to reveal the
plasma distribution.
