1. The problem statement, all variables and given/known data In a particluar x-ray tube, an electron approaches the target moving at 2.35 108 m/s. It slows down on being deflected by a nucleus of the target, emitting a photon of energy 40 keV. Ignoring the nuclear recoil, but not relativity, compute the final speed of the electron. 2. Relevant equations Equation for photon Energy: Ephoton = Ei - Ef 3. The attempt at a solution Ephoton = Ei - Ef Using the emitted photon energy as the change in energy (40 KeV = 6.409x10-15 Joules) and the relativistic equations for energy: E = pc = mevc. We have: 6.409x10-15 = mec(vi - vf) Where mec = (9.109-31)(3x108) Then we have: 2.345x107 = vi - vf We can use 2.35x108 as the electron's initial velocity: so vf = 2.35 108 - 2.345x107 = 2.1155x108. However, the answer is 2.27x108. So I am off by a small amount. What am I doing wrong?