05_02_04 - 22 Stimulated emission for balck-body radiation...

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Georg.Hoffstaetter@Cornell.edu 22 Einstein’s explanation from 1917 for the energy density in a black body box. The light in a black body is emitted by electrons that change their energy level: Equilibrium: Thermodynamic population of electron energy states: Stimulated emission for balck-body radiation Absorption Stimulated emission Spontaneous emission Level j Level i 1 / ) ( )] ( [ ) ( - = + = = j i ji ij ji ji N N B B B A ji ji j ij i i j j i u u B A N u B N N N ϖ ) ( u B N ji j ji j A N kT kT E E N N kT E j kT E i e e e C N e C N i j j i j i = = = = - - - , ) ( u B N N ij i j i = Assumptions to obtain Plank’s black-body radiation formula : Probability for stimulated emission = probability of absorbtion, Probability of spontaneous emission increases with
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05_02_04 - 22 Stimulated emission for balck-body radiation...

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