The Bremsstrahlung process
An ultra relativistic electron passing near a nucleus is influenced by its strong electric field. The interaction between electric charges is explained in modern physics by the exchange of photons. In (A), the electron exchanges a photon (not observed) with the nucleus. In (B), it emits a photon to be observed, the “braking” photon. The electron has lost energy, its trajectory has been deflected. If the electron does not pass very near the nucleus, the exchange is soft, so that the bremsstrahlung photon energy is low (infrared, visible, UV ...). On the contrary if the exchange is 'hard', the bremsstrahlung produces an X or gamma ray.