The Negative Absorption of Radiation

@article{RamanTheNA,
  title={The Negative Absorption of Radiation},
  author={Chandrasekhara Venkata Sir Raman and Kariamanikkam Srinivasa Krishnan},
  journal={Nature},
  volume={122},
  pages={12-13}
}
IN Einstein's celebrated derivation of the Planck radiation formula, an equilibrium is considered to exist between three elementary processes: (1) a spontaneous emission from the atoms, (2) an absorption of energy by the atoms proportional to the energy density in the field, and (3) an induced emission of energy from the atoms, also proportional to the energy density. The third process can be described as a negative absorption of radiation, and is quite characteristic for Einstein's theory, as… 
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