In a Coolidge tube, the energy of a characteristic X-ray photon is derived from

Step-by-step solution

Characteristic X-rays are produced when an inner-shell electron of a target atom is knocked out by a high-energy incident electron, creating a vacancy in the inner shell. An electron from a higher energy level of the target atom then transitions to fill this vacancy. The energy of the emitted characteristic X-ray photon is exactly equal to the difference in energy between these two atomic levels. Therefore, the energy of a characteristic X-ray photon is derived from an atomic transition within the target.