## Zur Theorie der Lichterzeugung und Lichtabsorption AND Das Prinzip von der Erhaltung der Schwerpunktsbewegung und die Trägheit der Energie in Annalen der Physik, Vol. 20, 1906, pp. 199-206; pp. 627-633.

Leipzig: Barth, 1906. 1st Edition. FIRST EDITION, FIRST ISSUE of two important 1906 Einstein papers. Einstein wrote two papers on the photoelectric effect, his revolutionary 1905 paper and "Zur Theorie der Lichterzeugung und Lichtabsorption," his continuation of it. In them, Einstein employed Planck's theory that luminous energy can be absorbed or emitted only in discrete amounts (called quanta) and proposed a theory of light quanta involving particles with no mass (photons) whose energy depended on frequency. All of Einstein's experimental results confirmed that light actually consisted of discrete energy packets.

"Based on this theory, Einstein wrote an equation describing how the photoelectric effect works. The energy of individual electrons emitted by a photocell is a function of the frequency of the light hitting the photocell, and the rate of electron emission is a function of the light source's intensity (number of photons with sufficient energy being emitted). This is contrary to what is predicted by classical physics" (History of Physics: The Wenner Collection).

In this, Einstein's second paper on photoelectrics, he revisited Planck's theory and from it, developed his ideas to show that an electromagnetic wave such as light could be described as a particle (photon) with discrete quanta of energy that was dependent on its frequency. In the long history of quantum mechanics, this would lead to a theory of unity between subatomic particles and electromagnetic waves called wave-particle duality in which particles and waves were neither one nor the other, but had certain properties of both.

At first Einstein believed that light-quantum hypothesis was merely 'heuristic': that it behaved only as if it consisted of discontinuous quanta. But in this paper and others to follow, Einstein used his statistical mechanics to demonstrate that when light interacts with matter, Planck's entire formula can arise only from the existence of light quanta -- not from waves. In other words, in explaining the photoelectric effect by extending Planck's concept of quantum of energy, had Einstein "demonstrated that his own 'light-quantum hypothesis' was implicit in Planck's earlier work" (Honner, The Description of Nature, 31).

ALSO included in this volume is "Daz Prinzip von der Erhaltung..." (The Principle of Conservation of Motion of the Center of Gravity and the Inertia of Energy). In this "ingenious thought experiment involving energy transport in a hollow cylinder, Einstein returned to the relationship between inertial mass and energy, giving more general arguments for their complete equivalence" (Calaprice, The Einstein Almanac, 18). This was the first statement that the conservation of mass is a special case of the conservation of energy. Item #292

CONDITION & DETAILS: Leipzig: Barth, 1906. Octavo. (8.75 x 6 inches); 222 x 152mm. Ex-libris bearing minimal markings (only a small stamp on the title page). Illustration: 6 plates and figures throughout. Entire volume in black cloth, gilt-lettered at the spine. The cloth is a bit rubbed and scuffed and there is fading at the spine. Solidly and tightly bound. Bright and clean throughout.

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