  The main features of radiation by relativistic electrons are well
approximated in the Weizsacker-Williams method of virtual quanta. This method
is best known for its application to radiation during elementary particle
collisions, but is equally useful in describing "classical" radiation emitted
during the interaction of a single relativistic electron with an extended
system, such as synchrotron radiation, undulator radiation, transition
radiation and Cerenkov radiation.
