Laser

How laser works?. The yellow photons represent the incident radiation and the red ones the laser radiation. Atoms are represented by the bluish spheres.

Photons excite atoms from the ground state to the highest energy state. This state is transitional. After a while, the atoms fall to the metastable state and stay there for a longer period of time.

When all the atoms reach the metastable state the laser beam is emitted. what follows is the return of the atoms to the ground state.