Aufnahme und Abstrahlung von Energie am Atom: This applet illustrates the absorption and emission of photons by an atom. An electron revolving around the nucleus may capture an incident photon. Once that occurs, the electron is raised to a higher energy level and thus changes its orbit to a one with a larger radius. While being in the excited state, the electron reemits the photon after some time and returns to the ground state. Note that one can observe the phenomenon involving the emission of the photon and its immediate recapture upon change of orbit (virtual photon emission).