  Radio signals are delayed when propagating through plasma. This type of delay
is frequency-dependent and is usually used for estimating the projected number
density of electrons along the line of sight, called the dispersion measure.
The dense and clumpy distribution of plasma can cause refractive deflections of
radio signals, analogous to lensing effects. Similar to gravitational lensing,
there are two contributions to the time delay effect in plasma lensing: a
geometric delay, due to increased path length of the signal, and a dispersive
delay due to the change of speed of light in a plasma medium. We show the delay
time for two models of the plasma distribution, and point out that the
estimated dispersion measure can be biased. Since the contribution of the
geometric effect can be comparable to that of the dispersive delay, the bias in
the measured dispersion measure can be dramatically large if plasma lensing
effects are not taken into account when signals propagate through a
high-density gradient clump of plasma.
