Advanced Undergraduate Quantum Mechanics pp 497-568 | Cite as

# Emission and Absorption of Light

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## Time-Dependent Perturbation Theory

It is no secret that quantum mechanics grew up to a large extent out of efforts to understand emission and absorption spectra of atoms. Indeed, the famous Planck distribution was introduced to explain the spectrum of the black-body radiation (not to be confused with black hole, of course), and one of Bohr’s postulates dealt explicitly with conditions for the emission or absorption of light by atoms. It is not surprising, therefore, that one of the first problems studied by Paul Dirac in his seminal 1926 paper on “new” quantum mechanics

^{1}(as opposed to the pre-1925 “old” quantum theory based on Bohr–Sommerfeld quantization principle) was the problem of interaction between light and atoms. This problem belongs to a broad class of problems, in which the Hamiltonian of a system can be presented as a sum of the “unperturbed” Hamiltonian \(\hat {H}_{0}\)## Copyright information

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