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.

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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.

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