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.