The Balmer Series
(section adapted from an optional historical note in Lsun5wav.htm)
The first clue to the meaning of spectral lines came in 1885 from Johann Balmer, a high school math teacher in Basel, Switzerland. Of all the atomic spectra, the simplest is that of hydrogen--not surprising, since hydrogen is the smallest of atoms, and presumably the simplest. In an electric discharge (similar to that of a neon lamp) it emits just one series of spectral lines, of which four usually register on the photographic record of a spectrograph (they were labeled by the first 4 Greek letters, as in the image above-- α, β, γ and δ, in order of descending wavelength). As noted, the wavelength λ (lambda) of each emitted color had been quite accurately measured, and Balmer discovered that the values all fit a simple formula:
1/λ = R [1/4 – 1/n2]
where n = 3,4,5.. and R is the experimentally obtained "Rydberg constant," named for Johann Rydberg, a Swedish physicist who evaluated it. (A translation of Balmer's original article exists on the web.)
The lowest of these "lines" (n=3) is the red "hydrogen alpha" line (Hα for short), responsible for the dominant red color of the solar chromosphere. Most sunlight originates in the photosphere, the outermost layer of the sun as seen by the eye; light in deeper layers is just reabsorbed near the place where it was emitted. The next layer out, the chromosphere, glows very dimly in red, a glow originally seen only during a total eclipse of the Sun, when the Moon blocked the much brighter light of the photosphere.
The chromosphere emits relatively little light, and its contribution is normally drowned out by the much greater brightness of the photosphere. It becomes visible during a total eclipse of the Sun. Then, after the Moon completely covers the photosphere, a reddish glow becomes visible around the Sun, in a relatively narrow ring; above it is the corona whose light is even fainter.
The chromosphere is important because it is the site of sudden energy releases associated with sunspot magnetism--the so called solar flares. Flares only rarely show themselves in white light, as brightenings against the background of the photosphere (one such rare event was the first flare to be observed, seen by Richard Carrington in 1859). But they are easily seen when viewed through a sensitive filter which only transmits the narrow Hα line and blocks everything else. Through such filters, flare activity and many other solar phenomena are regularly monitored and photographed.
After Balmer announced his series, Lyman found in the ultra-violet a series of lines1/λ = R [1 – 1/n2]
of which the "Lyman α" line is particularly prominent in the glow of the Earth's outer atmosphere, photographed by astronauts from the Moon. Also, Paschen found a series of lines in the infra-red1/λ = R [1/9 – 1/n2]
suggesting they all belonged to a single family with
1/λ = R [1/n2 – 1/m2] (
m>n, n,m = 1,2,3...)
The regularities of these series seemed like a clue to processes inside the atom, responsible for the emission of narrowly defined colors, or "spectral lines." But what was the message?