  We study gravitational lensing when plasma surrounds the lens. An extra
deflection angle is induced by the plasma in addition to the deflection
generated by gravity. An inhomogeneous plasma distribution generates a greater
effect than a homogeneous one, and may cause significant effects to be detected
in low frequency radio observations (a few hundred MHz). In particular, the
lensed image positions will be different between optical and radio
observations. The change of position due to a plasma can reach a few tens of
milli-arcsec, which is readily detectable. One can use the position difference
in different frequencies to estimate the density of plasma in the lens. The
magnification ratios between multiple images are mainly determined by other
properties of the lens, and are only weakly affected by the plasma. More
importantly, we find that the strong lensing time delay will be affected by the
plasma. Estimation of the Hubble constant from the time delay in low radio
frequency observation may be slightly biased due to plasma in the lens.
Unfortunately, the ionosphere of the Earth strongly affects low frequency radio
observations. Thus our ability to detect the effect depends on how well we are
able to calibrate out the ionosphere.
