This java applet is a quantum mechanics simulation that shows electric dipole transitions, which are interactions of classical electromagnetic radiation with an atom. It demonstrates absorption and stimulated emission.
When the applet starts up, there is an incoming wave which is at the right frequency (the Bohr frequency) to transition the particle from the ground state to 2px. Once it reaches that state, it transitions back to the ground state, and so on.
To view another transition, make a selection from the popup menu in the upper-right.