This applet illustrates a schematic operation of a laser. The yellow photons represent
the pumping radiation. The group of red
photons is the coherent laser beam. The balls mark the atoms making transitions
between three energy levels.
The pumping radiation couses the transition of atoms from the ground state to the
high energy excited state. From this short-living state the atoms go by non-radiative
transition to the long-living metastable state. Once in the metastable state many
atoms can be accomulated. The laser beam, stimulated emission, arises when all atoms
simultaneously make a transition to the ground state.