Even above the cutoff frequency there are various propagation
effects that are important for a radio wave passing through a plasma.
Let us start with the most straightforward ones. Consider a radio signal
passing through a homogeneous slab of plasma of length L. The signal
is delayed (with respect to the propagation time in the absence of
the plasma) by the amount
The propagation delay can also be considered as an ``excess path length''
= c . Further since ( and differ
only in sign16.1, the magnitude of the ``excess phase'' (viz.
is given by
. Note that since the propagation
delay is a function of frequency , waves of different frequencies get
delayed by different amounts. A pulse of radiation incident at the far end
of the slab will hence get smeared out on propagation through the slab;
this is called ``dispersion''. If the plasma also has a magnetic field
running through it then it becomes birefringent - the refractive index is
different for right and left circularly polarized waves. A linearly
polarized wave can be considered a superposition of left and right circularly
polarized waves. On propagation through a magnetized plasma the
right and left circularly polarized components are phase shifted by different
amounts, or equivalently the plane of polarization of the linearly
polarized component is rotated. This rotation of the plane of polarization
on passage through a magnetized plasma is called ``Faraday rotation''.
The angle through which the plane of polarization is rotated is