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The Loudon quantum theory of light postulates that light is composed of quantized particles called photons, which are the quanta of the electromagnetic field. The theory describes the behavior of photons in terms of their wave-like and particle-like properties.
[1] R. Loudon, "The Quantum Theory of Light," Proc. R. Soc. London A 274, 471 (1963). loudon quantum theory of light pdf
[2] R. Loudon, "Quantum Optics," Oxford University Press, 2000. The Loudon quantum theory of light postulates that