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In this section, we describe
the absorption coefficient and the emissivity
for the line emission and absorption.
Here, consider a hypothetical atom which has only three levels
named n, n, n.
and assume the energy of each level is, respectively, , ,
When the atom changes its state from n to n,
an photon with energy of
if this transition is radiative.
Besides the radiative processe, atom changes its state exchanging
its energy with other atoms, molecules and electrons.