**Einstein Coefficient Relation derivation and discussion:
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Einstein showed the interaction of radiation with matter with the help of three processes called stimulated absorption, spontaneous emission and stimulated emission. He showed in 1917 that for proper description of radiation with matter,the process of stimulated emission is essential.Let us first derive the Einstein coefficient relation on the basis of above theory:

Let N_{1} be the number of atoms per unit volume in the ground state E_{1} and these atoms exist in the radiation field of photons of energy E_{2}-E_{1} =h v such that energy density of the field is E. Continue reading “Einstein Coefficient Relation”