Spectroscopy

It has been known since before humans were recording observations that very hot things glow. As we began to separate elements, we discovered that different elements glowed with different colors.

By the mid to late 1800's we knew that the colored light was not generally a single color, but rather a combination several colors. These colors could be separated and studied by passing the light through a prism or a diffraction grating. As our precision improved, we were able to measure the wavelengths of the various colors of light.

Hydrogen, as the simplest element, was studied early on. A tube of hydrogen gas, when excited with electricity will emit a pinkish light. That light, when separated, turns out to be 4 colors of visible light: red, teal, violet and deep purple.

The Balmer Series

In 1885, Johann Balmer, a Swiss mathematics instructor, was studying the hydrogen spectrum. More specifically, he was studying the wavelengths of these 4 spectral lines.

After playing around with the values, Balmer determined that the wavelength of each line could be related according to the following equation:

Where λ is the wavelength of the particular color of light, R_H is a constant (called the Rhydberg constant) and n is either 3, 4, 5, or 6.

Between 1906 and 1914, Theodore Lyman discovered that the equation could be re-worked to give the wavelengths of the ultraviolet (UV) light emitted by the hydrogen tube:

where n = 2, 3, 4, 5, or 6.

In 1908, Friedrich Paschen discovered that the equation could also be made to fit the infra-red (IR) light emitted by hydrogen by adjusting it to be:

where n = 4, 5, or 6.

Determining what all of this meant would require the work of J.J. Thomson and Neils Bohr.