What Is the Early History of Spectroscopy?
Scientists of antiquity often investigated everyday matters in an attempt to unravel their apparent universe. Such a study is where the roots of spectroscopy lie, when in the 1200s people started to look at how rainbows form. Everyone’s favorite Renaissance Man Leonardo da Vinci tried to replicate a rainbow using a globe filled with water and placing it in sunlight, noting the patterns in the colors. In 1637 Rene Descartes wrote Dioptrique where he talks about his own rainbow studies using prisms. And in 1664 Robert Boyles Colours used an updated rigging like Descartes in his own study (Hirshfeld 163).
All of this led Newton to his own research in 1666, where he set up a dark room whose only light source was a light hole that shined into a prism, thus creating a rainbow on the opposite wall. Using this tool, Newton comes upon the idea of a spectrum of light, where colors combine to make white light and that the rainbow could be broadened out to reveal even more colors. Further refinements in the following years saw people almost hitting upon the true nature of the spectrum when in the mid-1700s Thomas Melville noticed that the Sun’s flares had a different intensity to their spectrum. In 1802 William Hyde Wollaston was testing the refractive properties of translucent materials using a slit of light 0.05 inches in width when he noticed the Sun had a missing line in the spectrum. He didn’t think this was a big deal because no one felt the spectrum was continuous and that gaps would be present. So close they were to figuring out that the spectrum held chemical clues (163-5).
Instead, the birth of solar and celestial spectroscopy happened in 1814 when Joseph Fraunhofer used a small telescope to magnify sunlight and found that he wasn’t satisfied with the image he was getting. At the time, mathematics were not practiced in lens making and instead one went by feel, and as the size of the lens increased so did the number of errors. Fraunhofer wanted to try and use mathematics to determine the best shape for a lens and then test it out to see how his theory held up. At the time, multielement achromatic lens’ were in vogue and were dependent on the make up and the shape of each piece. To test out the lens, Fraunhofer needed a consistent light source to be a basis for comparison, so he employed a sodium lamp and isolated certain emission lines he saw. By recording the changes in their position, he could gather properties of the lens. Of course, he was curious as to how the Sun’s spectrum would fair with this rigging and so turned its light onto his lenses. He found that many dark lines were present and counted 574 in total (Hirchfield 166-8, “Spectroscopy”).
He named then Fraunhofer lines and theorized that they originated from the Sun and were not some consequence of his lenses nor of the atmosphere absorbing light, something that would later be confirmed. But he took things further when he turned his 4-inch refractor with prism at the Moon, planets, and various bright stars. To his amazement, he found that the light spectrum he saw was similar to the Sun! He theorized this was because they reflected the Sun’s light. But as for the stars, their spectrums were very different, with some portions brighter or darker as well as different pieces missing. Fraunhofer set the bedrock for celestial spectroscopy with this action (Hirchfield 168-170).
Bunsen and Kirchhoff
By 1859, scientists continued this work and found that different elements gave different spectrums, sometimes getting a nearly continuous spectrum with missing lines or an inversion of that, with a few lines present but not much there. In that year though, Robert Bunsen and Gustav Kirchhoff figured out the secret of these two, and it comes in their names: emission and absorption spectrums. The lines only were from an element being excited while the nearly continuous spectrum came from the light being absorbed in the spectrum of an intermediary light source. The position of the lines in either spectrum was an indicator of the element being seen, and could be a test as to the material that was being observed. Bunsen and Kirchhoff took this further though when they wanted to set up specific filters in an attempt to help in further properties by removing the light from spectrums. Kirchhoff investigated what wavelengths were located, but how he did this is lost to history. More than likely, he utilized a spectroscope to break down a spectrum. For Bunsen, he had difficulties in his efforts because differentiating different light spectrums is challenging when the lines are so close to each other, so Kirchhoff recommended a crystal to further break the light up and make it easier to see the differences. It worked, and with several crystals and a telescopic rig Bunsen started to catalogue different elements (Hirchfield 173-6, “Spectroscopy”).
But finding elemental spectrums wasn’t the only finding that Bunsen made. In looking at spectrums, he discovered that it just takes 0.0000003 milligrams of sodium to really affect a spectrum’s output because of its strong yellow lines. And yes, spectroscopy yielded many new elements unknown at the time, like cesium in June of 1861. They also wanted to use their methods on stellar sources but found that frequent flaring from the Sun caused portions of the spectrum to disappear. That was the big clue to absorption vs. emission spectrum, for the flare was absorbing the portions that disappeared briefly. Remember, this was all done before the theory of atoms as we know it was developed, so it was all attributed solely to the gases involved (Hirchfield 176-9).
Kirchhoff continued his solar studies but he ran into some difficulties that were mainly a result of his methods. He chose an “arbitrary zero-point” to reference his measurements, which could change depending on what crystal he was using at the time. This could alter the wavelength he was studying, making his measurements prone to error. So, in 1868 Anders Angstrom created a wavelength based solar spectrum map, thus providing scientists with a universal guide to spectrums seen. Unlike the past, a diffraction grating with set mathematical properties was referenced as opposed to a prism. In this initial map, over 1200 lines were mapped! And with the advent of photographic plates on the horizon, a visual means of recording what was seen was soon upon everyone (186-7).
Hirshfeld, Alan. Starlight Detectives. Bellevine Literary Press, New York. 2014.Print. 163-170, 173-9, 186-7.
“Spectroscopy and the Birth of Modern Astrophysics.” History.aip.org. American Institute of Physics, 2018. Web. 25 Aug. 2018.
© 2019 Leonard Kelley