## Zur Quantentheorie der Strahlung [On the Quantum Theory of Radiation]

First printing in *Physikalische Zeitschrift*, the first formal journal publication (following the very rare 1916 appearance in *Mitteilungen der Physikalischen Gesellschaft Zurich*) of Einstein's development of his derivation of Planck's law and the theoretical basis for the laser. Weil *91.

"Einstein used Bohr's model as the foundation for a series of papers in 1916, the most important of which, 'On the Quantum Theory of Radiation,' was also formally published in a journal in 1917 [*Physikalische Zeitschrift*]. Einstein began with a thought experiment in which a chamber is filled with a cloud of atoms. They are being bathed by light (or any form of electromagnetic radiation). Einstein then combined Bohr's model of the atom with Max Planck's theory of the quanta. If each change in an electron orbit correspond to the absorption or emission of one light quantum, then--presto!--it resulted in a new and better way to derive Planck's formula for explaining blackbody radiation. As Einstein boasted to Michele Besso, 'A brilliant idea dawned on me about radiation absorption and emission. It will interest you. An astonishingly simple derivation, I should sway *the* derivation of Planck's formula. A thoroughly quantized affair.'

"Atoms emit radiation in a spontaneous fashion, but Einstein theorized that this process could also be stimulated... This process of stimulated emission would, almost forty years later, be the basis for the invention of the laser, an acronym for 'light amplification stimulated by emission of radiation'" (Isaacson, *Einstein*).

IN: Physikalische Zeitschrift, Vol. 18, pp. 121-128. Leipzig: S. Hirzel, 1917. Early three-quarter cloth over contemporary marbled boards, marbled endpapers. Whole volume (1917) offered. Light wear to binding, otherwise fine. Weil *91.

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