Demonstration, The Laser

The Laser
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.