In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses a mystery scientists encountered when they studied light and used prisms to create spectra: what was causing those dark lines? – Kim
By Dean Zollman
In recent posts I have been discussing the progress in understanding the relations among the elements. The evidence provided by the categories of the elements of properties and by the periodic table offered a lot of evidence that atoms existed. However, for many 19th century scientists, none of these discoveries was the “smoking gun” that conclusively proved atoms existed. By the early 20th century most of these discoveries would be explained in terms of atoms, but for many 1870s scientists, they were just interesting and useful results.
As with most fundamental scientific discoveries, the road to atoms had multiple lines of reasoning and discoveries. In this and the next posts, I will discuss another thread that started at the beginning of the 19th century and was not thoroughly explained until the first part of the 20th century. So we will back up in time to the early 19th century and follow another thread – light emitted by different substances.
Light emitted by matter is a rather common phenomenon. It is most evident in daily life as light from the sun, from burning objects, or coming from many different kinds of lamps. The ancients learned that the color of the flame varied with the type of material that was burning and that the color changed with the temperature. (The latter observation has been very important to potters for thousands of years.)
Until Isaac Newton, no one had systematically studied the light from objects. When Newton passed sunlight through a prism, he saw the light was separated into the colors of rainbow. He called this display of colors a spectrum. And the name has stuck ever since. Newton explained how a prism could form a spectrum in his Optics, but he did not investigate the spectrum itself further. (Newton’s explanation was not correct, but that’s a different story from the one that I want to tell.)
Today, we can see spectra, for example, in rainbows and in the light reflected off a DVD. That display of colors you see when looking at a DVD is the spectrum of the light that is reflecting from it. If you would like to use an old CD to make your own instrument to look at spectra, these videos will help:
However, do not look directly at the sun with any device. To view the sun’s spectrum, bounce sunlight off white paper. Otherwise, you will damage your eyes.
The serious study of spectra began near the end of the 18th century. By 1801, experiments had established that the solar spectrum extended beyond visible light and included both the infrared and ultraviolet. In 1802, William Hyde Wollaston (1766-1828) was investigating which primary colors were present in the solar spectrum. His apparatus was somewhat better than those before him. As a result he saw an important detail in the spectrum that others had not seen – dark lines. While Wollaston reported the dark lines, he was interested in the colors in the spectrum, so he did not pursue the lines further.
Wollaston also studied the spectra of light from other sources. He passed light from candles and electrical sparks through the prisms. While doing so, he saw spectra and some bright lines of various colors. But neither he nor his contemporaries pursued an understanding of these spectra.
In 1814, Joseph von Fraunhofer (1787-1826) rediscovered the lines. He cataloged 574 dark lines in the sun’s spectrum. One of the most important was a line he labelled D and was in the yellow portion of the spectrum. He also looked at the spectra of terrestrial lights and noted that these lights had a bright yellow line at the same place as the dark solar D line. During his investigations he discovered that both the dark D lines and the bright yellow line from lamps were really two closely spaced lines (now called a doublet). This was a strong hint that something that was happening with light coming from the sun was similar to something that was happening on light emission on earth. But it would take quite a while for scientists to figure out the connection.
The spectrum below is similar to the one Fraunhofer saw, but this one has been modified to make the dark lines more apparent. It also has many fewer lines than Fraunhofer saw and far fewer than have been discovered now. The labels A, B, and so on were created by Fraunhofer. Today, these lines are called the Fraunhofer lines.
The connection between the dark lines in the solar spectrum and materials on earth would need to continue for quite a while. One line of investigation was the study of light from burning salts. Investigators found that different salts burned with different colors and displayed different spectra. Two significant issues were that many of the spectra were quite complex and that the bright yellow lines corresponding to the Fraunhofer D lines seemed to be in almost all materials.
The idea that Fraunhofer’s lines were connected to processes on earth gained some experimental support. In 1842, Daniel Brewster (1781-1868) reported to the British Association for the Advancement of Science that he had investigated the red spectrum of saltpeter and found that “all the black lines of Fraunhofer were depicted in the spectrum in brilliant red.”
During this same time period the spectra of “electric light” was being investigated. Electric light was created by applying a high voltage to two electrodes of the same metal. The sparks between the electrodes produced light which could be passed through a prism. In this way the spectra of metals could be studied.
Charles Wheatstone (1802-1875), among others, discovered that each metal had a characteristic spectrum. He even set up situations where each of the two electrodes were different metals. Then, he saw the spectra of both metals. However, he missed an important point. About 15 years later, Antoine Masson discovered that the spectrum of electric light contained some common lines independent of the metal as well as those unique to each metal. Masson did not quite carry his investigation far enough, so he also missed an important point.
It fell to Anders Ångström (1814-1874) to discover that the spectrum of an electric light depended on both the metal and the gas surrounding the metal. The common component that Masson saw was the spectrum of air. Thus, Ångström was the first to establish that gases also emitted light that could be analyzed with a prism. As we shall see, these gas emission spectra would prove to be critical to progress in understanding atoms well into the 20th century. (If you did spectral analysis of gases in a chemistry or physics class, you used equipment that was similar to that of Ångström.)
Some spectra with dark lines were also created in the labs. Experimenters would pass full spectrum light through a gas or an arc of an electric light and view the spectrum of the result. Some dark lines would appear.
With all of this information, an adequate explanation of the dark lines in the solar spectrum eluded scientists for about a half of a century. While they suspected that there must be a relation between the dark lines and emission spectra, scientists were hampered because they had no clear model for emission of light and had yet to appreciate fully that each type of atom had a unique spectrum. In fact, some were still arguing about whether atoms exist.
And then that doublet line in the yellow, Fraunhofer’s D, was perplexing. It seemed to be almost everywhere any experimentalist looked.
So, progress was slow, but there was progress. Next time we will look how a physicist and chemist worked together to provide an explanation for this puzzle. Their work would eventually lead to a mathematical formula discovered by a high school math teacher, and in the 20th century, a model of the atom that can be used to derive that formula. It will be a while before we get there because this puzzle has a lot of pieces that we have not looked at yet.
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.