The Hydrogen Spectrum

Go back and read through A Brief History of Spectroscopy up to 1862 if you haven't done so already.

An emission spectrum of an element has lines in it which are against a black background. If those lines are in the visible spectrum, then we see them as specific colors. Outside the visible spectrum, the lines only appear as lines on a photograph.

The emission spectrum of hydrogen occupies a very important place in the history of chemistry and physics. Niels Bohr, in 1913, will use the hydrogen spectrum to start on the road to explaining how electrons are arranged in an atom.

Here is a drawing of the visible spectrum of hydrogen:

The left edge ends at a wavelength of 4000 Ångströms and the right edge ends at 7000 Ångströms.

(The Ångström is equal to 10¯8 cm. and its symbol is Å. It was established by Anders Ångström and he named it after himself. It is not an official SI unit, but its use is tolerated. The more modern wavelength unit is nanometers, symbol nm. 4000 Å = 400 nm, 7000 Å = 700 nm.)

The four colored lines are the four visible lines in the hydrogen spectrum. Their wavelenghts in Ångströms are shown in the diagram. There is one red line (6562.852 Å), one blue-green line (4861.33 Å), and two violet lines (4340.47 Å and 4101.74 Å).

In 1862, Anders Ångström discovered three lines and later on found the 4th line (the 4101.74 violet line). By 1871, he measured all four wavelengths to a high degree of accuracy.

This next diagram is a photograph of a more complete hydrogen spectrum:

Hα, Hβ, Hγ, and Hδ are the official designations for the 4 lines of the visible portion of the spectrum. All the other lines to the left of these four are in the ultraviolet portion of the spectrum and so would not be visible to the eye. However, they can be photographed, as they were here.

This grouping of lines has a name. It is called the Balmer Series. In 1885, Johann Balmer was able to calculate the four visible lines' wavelengths using one formula, now called the Balmer Formula. Notice how the lines get closer and closer to each other. This will become important later.