  The effect of ionized gas on the propagation of radio signals is known as
plasma lensing. Unlike gravitational lensing, plasma lensing causes both
magnification and strong de-magnification effects to background sources. We
study the cross section of plasma lensing for two density profiles, the
Gaussian and power-law models. In general, the cross section increases with the
density gradient. Radio sources can be used to measure the free electron
density along the line of sight. However, plasma lensing de-magnification
causes an underestimate of the electron density. Such a bias increases with the
electron density, and can be up to $\sim 15\%$ in the high density region.
There is a large probability that high density clumps will be missed due to
this bias. The magnification of plasma lensing can also change the luminosity
function of the background sources. The number density of sources on both the
high and low luminosity ends can be overestimated due to this biasing effect.
