Absorption and Emission of Radiation by an 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).