  The square of the four momentum of a photon in vacuum is zero. However, in an
unmagnetized plasma it is equal to the square of the plasma frequency. Further,
the electron-photon coupling vertex is modified in a plasma to include the
effect of the plasma medium. I calculate the cross sections of the three
processes - the Compton scattering, electron-positron pair annihilation and
production in a plasma. At high plasma densities, the cross sections are found
to change significantly. Such high plasma densities exist in several
astrophysical sources.
