Under normal circumstances when light interacts with matter, electrons in the matter may absorb the light energy and go to the higher energy level and come back to the ground state by emitting the light of the same frequency. But in any given situation the number of electrons in the ground state is more than that in the excited state. The Laser is based on the principle of light amplification by stimulated emission of radiation. This is achieved by having excess concentration of electrons in the higher energy states and stimulating the system by radiation to bring about the de-excitation process