ABSTRACT
It is well known that the absolutely free electron (i.e. the electron which moves in the infinite vacuum and is not affected by any external field) does not radiate. To make him radiate, it is necessary to deprive him of the freedom: to replace the vacuum with a medium or to introduce an external (static or alternating) field. In quantum words, to be born, each photon needs two parents. This can be achieved in three ways using
an electron and a medium (Cherenkov and transition radiations),
an electron and a static field (Bremsstrahlung radiation),
an electron and an incident photon (wave scattering).
