In bremsstrahlung, the photon energy is equal to which of the following?

• A. Difference between the incoming and outgoing electron energy
• B. Energy of the incoming electron
• C. Energy of the outgoing electron
• D. Difference between inner and outer binding energies

Answer: (A)

In bremsstrahlung, a fast electron is decelerated by the electric field of a nucleus, and the energy it loses shows up as a photon. Energy must be conserved, so the photon’s energy equals the energy lost by the electron during the interaction—the difference between the electron’s incoming energy and its outgoing energy after the encounter. The electron often still has some kinetic energy after emission, so the photon energy is not simply the incoming energy and can vary continuously up to nearly that amount. The concept of binding energy differences applies to atomic transitions, not to bremsstrahlung radiation from deceleration. So the photon energy is the energy lost by the electron, the difference between its initial and final energies.