The Bohr model of an atom nicely explained the experimental laws of spectroscopy discovered by Kirchhoff. In thin hot gas atoms collide with each other, raising electrons to higher-level orbits. Soon they drop back downward to lower-level orbits. As a result, the atom radiates photons whose energy corresponds to the energy difference of the orbits. Thus the spectrum of the gas shows bright emission lines (Kirchhoff's II law). When radiation passes through thin gas, those photons which have the right energy to raise an electron from a low orbit to a higher orbit are absorbed. Thus absorption lines are created in exactly the same places in the spectrum where the bright emission lines appear (Kirchhoff's III law). In dense gas and in solids, the atoms are so close together that they perturb each other's electron orbits; the orbits are shifted from their usual orbital radii. As a result, energy jumps of all kinds can appear, and photons of all wavelengths are emitted. Thus a continuous spectrum is observed (Kirchhoff's I law).
Bohr was appointed professor of theoretical physics at Copenhagen in 1919. A special institute was founded to further his research; it became one of the leading centers of the study of atomic physics, a place where researchers from different parts of the world could meet, not always easy in the post-World War I atmosphere.1
Bohr's model explained the radiation of atoms so well that gradually it was accepted as fact (see Box 17.1). But the assumptions made by Bohr had no real basis in physics. Many of the physical laws in the microworld are quite different from the laws found in our usual environment. Neither Newton's mechanics nor Maxwell's electromagnetic theory could be directly applied to the phenomena at the atomic level.
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