Robert McNees
Danish physicist Niels Bohr was born #OTD in 1885.
Bohr applied the nascent quantum theory to the structure of the Hydrogen atom, offering an explanation (though not a true mechanism) for an outstanding problem that classical physics could not account for.
Image: Atomic Heritage Foundation
Robert McNees
In 1911, scattering experiments by Ernest Rutherford and collaborators had disproven the “plum pudding” model of the atom.
Rather than a blob-like structure with + and - charge spread throughout, experiment indicated atoms have a tiny positive nucleus.
Negative electrons must “orbit” this nucleus. But classically such a configuration is not stable! The accelerating electrons would radiate energy and spiral into the nucleus.
Rutherford’s atom seemed incompatible with known physics!
Robert McNees
Classical physics was already having problems with atomic phenomena.
Atoms were known to emit and absorb light preferentially at certain wavelengths. The "spectral lines" fit simple patterns but had no known explanation.
Image: Emission spectrum of Hydrogen
Robert McNees
Bohr addressed both issues at once by hypothesizing the existence of "stationary orbits" around the nucleus.
Electrons could exist in these stationary orbits without (as expected from classical electrodynamics) radiating energy.
Transitions between these orbits would emit energy in discrete amounts (related to Planck's quanta) dictated by the initial and final level. This reproduced the empirical formulas obtained by Balmer and Rydberg for the wavelengths of the spectral lines.
Robert McNees
Bohr's model was purely phenomenological. It was a hodgepodge of early quantum ideas that fit the data without offering any deeper explanation.
It also preceded de Broglie's hypothesis, which would have allowed Bohr to identify these orbits as integer multiples of an electron's wavelength.
But it seemed to resolve –– or at least relate –– two outstanding puzzles that had baffled scientists. So he was clearly onto something.
Robert McNees
Bohr’s model is a pillar of the "old quantum theory" – the transitional body of work straddling classical physics and modern quantum mechanics. These early results, which explicitly called for new physics, set physicists on the road to our present understanding of Nature at the atomic and sub-atomic level in terms of quantum mechanics.
Lee
@mcnees And he had NOT started to shave.
Level 98
@mcnees Yes. This is interesting because it's usually presented at a high school level as Bohr using de Broglie's hypothesis. I mean, I present it this way, but I DO mention that' I'm simplifying the story (as we usually do in teaching) for time / ease of learning.
Bohr "quantised" the angular momentum rather than "fitting" waves into orbits.
However, IMO, it's a great example of what "semi-classical" means and, more generally, a great example of transitioning from one "paradigm" to another.
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