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1862 Anders Jonas Ångström identified three lines in the visible portion of the hydrogen emission spectrum, a red line, a blue-green line and a violet line. Later on, he identifies the violet line as being two lines very close together.
1868 Ångström introduced a great improvement. In his book Recherches sur le spectre solaire, which contains an atlas of the solar spectrum with the wavelengths of over a thousand lines obtained by the use of diffraction gratings, he expressed his wavelengths in units of 10¯10 m. Before this time scales with arbitrary units had been used. This Ångström unit is still widely used.
Ångström measured the wavelengths on the four visible lines of the hydrogen spectrum. In modern values, they are:
James Clerk Maxwell, a Scottish physicist, suceeded in unifying electricity and magnetism. He did this by showing that the two forces have a common origin. Since that time, we speak of electromagnetism. In his theory, it was clear that the emission of radiation by matter must be as a result of the acceleration of electric charges. It seemed that within matter there were mobile electrical charges that, as they moved in some fashion, produced the spectrum.
However, here is where his theory failed. It predicted that the spectrum could be resolved into sets of spectral lines, each consisting of a fundamental frequency (let's call it nνo) and its harmonics. The fundamental would be when the n = 1 and the harmonics would be where n = 2, , 4, . . . . In no case did the spectral lines correspond to what the theory predicted. The (predicted, but still undiscovered in 1873) mobile charges DID NOT obey the rules that Maxwell had discovered.
[I hope it is clear to my readers that these mobile charges are now called electrons and that there is a set of rules the electrons obey. Bohr, in 1913, will begin the process of discovering exactly what those rules were.]
1880s to 1908
During this time, many empirical regularities were found in spectral lines. What this means is that some spectral lines could be related to others by an equation, but no one knew why the equation worked. The most famous of this time was the Balmer formula in 1885.
Exactly what is the Balmer formula? It gave a simple, mathematical relationship between the wavelengths of the four lines of the visible spectrum of hydrogen, measured by Ångström in 1871. The equation shows that the four numbers are related in some way, but in 1885 no one knew what that relationship was.
Pieter Zeemen discovered the effect named for him: the Zeeman Effect. He discovered that spectral lines split into very closely spaced groups when the light was passed through a magnetic field first.
For example, the red line of the hydrogen spectrum was know to be 6562.852 Å, but that was with no magentic filed present between the hydrogen source and the measuring device (called a spectroscope). However, pass the light through a magnetic field on its way from source to instrument, that's a different story. That single red line split (I think into three, but I'm not sure) into several lines that were very closely spaced around the 6562.852 Å value.
Zeeman, of course, published the results of his work and others around the world began to look at other lines in the spectrum of hydrogen and other substances. It turns out that almost every line split in some fashion. Some split into three lines, others into five. Some split into two and others into two groups of two. And the individual lines in the resulting group did not all have the same intensity of color. Also, if the magnetic field was rotated 90 degrees, sometimes different splittings resulted.
All in all, it was very, very difficult to figure out what was going on. However, a correct explanation was eventually arrived at, although it took a number of years.
A point of interest: Zeeman (a native of Holland) received a promotion to a more well-known university as a result of this discovery. (He also won the Nobel Prize.) However, his new university had older and poorer equipment than his old job and he could not reproduce his experiment until better equipment was purchased.
Niels Bohr publishes an explanation of the hydrogen spectrum, which today we call "The Bohr Model of the Atom." In it, he uses the standard concepts of the time, but with a twist. He uses the "quantum hypothesis," introduced in 1900 by Max Planck and makes a critical assumption about how the lines in the spectrum are generated.
I will end my (incomplete) spectrum history here. There is more, much more, but that is a story for another day. However, just a taste:
(1) It turned out that there were splittings within the line splittings discovered by Zeeman. This resulted in the discovery of "spin" in 1925.
(2) It turned out that the lines were not exactly in the places predicted, but were offset by a tiny, tiny amount. This was demonstrated by Willis Lamb, who received a Nobel Prize for discovering the "Lamb Shift." The Lamb Shift is due to the electrons interacting with the nucleus and his 1949 experiment is considered a classic.
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