# Classical and Quantum Theories of Radiation

## Abstract

Quantum theory developed from the study of the interaction of light and matter. Around the beginning of this century, experimental physicists reported phenomena that demonstrated the inadequacy of classical physics. Quantum features of the electromagnetic field were first postulated by Planck in 1900 in order to account for the spectrum of blackbody radiation observed by Lummer and Pringsheim. In order to account for the discrete sequence of wavelengths in the spectrum of atomic hydrogen, Bohr in 1913 postulated that the electron could move only in certain stationary orbits; Planck’s constant appeared in his theory as the fundamental “unit of action.” We are all familiar with these and other inspired guesses which led finally to the birth of quantum mechanics, as we know it, in the years 1925–26. Dirac’s paper on the quantum theory of the electromagnetic field appeared in 1927,^{(1)} and by 1930 there was little doubt that the quantum theory of light and matter was vastly superior to classical theory. Born was so impressed by the theory that at the Fifth Solvay Conference in 1927 he remarked, “We consider that quantum mechanics is a complete theory, and that its fundamental hypotheses, both physical and mathematical, are not susceptible to further modification.”

## Keywords

Quantum Theory Spontaneous Emission Probability Amplitude Radiation Reaction Lamb Shift## Preview

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## References and Notes

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