To get the proper wavelengths to generate the chain reaction, first electricity or another energy source is ”pumped” into a chamber filled with a particular atoms or molecules. Then this “pumping” radiation causes the transition of atoms from ground state to higher energy excited state. From this shortlived state the atoms come down through non-radiative transition to the long-lived metastable state. Once in metastable state many atoms can be accumulated in one place and in the same state. The LASER or MASER beam, stimulated emission, arises when all these accumulated atoms simultaneously make transition to the ground state, releasing their energy of wavelength ? ,creating a beam of microwave radiation (or visible light in case of laser)which can be sent on the other atoms to cause the chain reaction. Since all the resulting photons have the same wavelength and the laser beams are extremely focused and coherent

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References

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