English: In a 1909 paper, Einstein presented this paradox to argue that atoms emit light as discrete particles rather than as continuous waves. An electron in a cathode ray beam strikes an atom in a target. The intensity of the beam is set so low that we can consider one electron at a time as impinging on the target. The atom emits a spherically radiating electromagnetic wave, which excites an atom in a secondary target, causing it to release an electron of energy comparable to that of the original electron. The energy of the emitted electron depends only on the energy of the original electron and not at all on the distance between the primary and secondary targets. All the energy spread around the circumference of the radiating electromagnetic wave would appear to be instantaneously focused on the target atom, an action that Einstein considered implausible. Far more plausible would be for to say that the first atom emitted a particle in the direction of the second atom.