By Don Lincoln, Fermilab
The invention of diffraction gratings revolutionized precision astronomy. We still use them today. One can see them in a CD or a DVD, in the way it reflects light. So, what exactly are diffraction gratings? What are their uses in astronomy?
German physicist Joseph von Fraunhofer built, what is called, a diffraction grating, a series of very closely spaced lines etched into a piece of glass. Diffraction gratings act like prisms, but much better. Prisms are subject to non-uniformities in the glass, resulting in small distortions in the spectrum. Diffraction gratings are much more precise and are what is needed for astronomical studies. One of the most serious advantages of this technology is its reproducibility, meaning that researchers at different laboratories will get the same result. With prisms, each prism might bend light by a slightly different amount. This would result in each researcher reporting different numbers for the wavelength of any features seen in the emitted spectrum of light.
CDs and DVDs work by inscribing a precise grid of dots into the disk and when they reflect light, the result is visually the same as a more precise and professionally made diffraction grating.
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In any event, in 1814, Fraunhofer was looking at the spectrum of sunlight and observed that sunlight was broken into a rainbow as he expected, but he also saw certain wavelengths of light that were missing. There were dark lines in the rainbow. Fraunhofer mapped out over 570 dark lines in the Sun’s spectrum; 11 that were especially dark, and the rest more minor in comparison. We still call these Fraunhofer lines.
Fingerprints are one way that we uniquely identify humans here on Earth. Every person has their own unique set. A similar thing is true with chemical elements. In addition to their various chemical properties, each element emits a specific set of wavelengths of light when they are heated enough that they glow. This was first recognized back in the mid-1700s, when scientists would take samples of chemicals, put them in a flame, and notice how the color of the light changed. For instance, copper salts often glow green when treated this way, and ordinary table salt glows especially brightly in the yellow.
It was in the 1850s that researchers realized hot gases emitted light of specific wavelengths. These were bright lines set against a black background. For instance, hydrogen has a couple of dark blue lines, a turquoise one, and a red one. Sodium has a couple of turquoise lines, a couple that are the color of Mountain Dew, two yellow—one of which is very thick and turns out to be two closely spaced lines—and an orange-red one. Other elements each emitted its own distinct lines of color. These are essentially fingerprints of the elements.
Electrons’ Transitioning Energy levels
It was in about 1860 that German chemist Robert Bunsen (of Bunsen burner fame) and physicist Gustav Kirchoff realized that the dark Fraunhofer lines and the light lines seen in specific elements, occurred at the same wavelength. So, what do these lines mean?
In modern times, we know much more than these astronomical pioneers did. We know that atoms of each element have a characteristic set of energy levels for the electrons surrounding them. When electrons go from a high energy level to a low energy one, they emit light of specific wavelengths.
Conversely, when light passes through the gas of a specific element and the wavelength is just right, the light can kick an electron from a low energy level to a high energy level. The light gets absorbed and the result is a black line. Thus, the bright lines are due to the emission of light by an element and the dark lines are caused by the absorption of light by the same element. In both cases, it is due to the transition of an electron from one energy level to another.
The Discovery of Helios
Of course, they didn’t know any of that back in the 1860s. They just knew that certain elements gave off specific spectral lines. And, of course, they spent lots of time cataloguing all the lines. That attention to detail paid off in 1868.
1868 was the year of what was called ‘The King of Siam’s eclipse’. French astronomer, Pierre Janssen, located in British India and British astronomer, Joseph Norman Lockyer, studied light emitted from the Sun’s corona and found a yellow line in the spectrum that was created by no known element. They hypothesized that this line was emitted by a new element.
The two men sent papers to the French Academy of Science and the papers arrived the exact same day. Thus, the two men were jointly awarded credit for the discovery of the element, named for Helios, Greek Titan of the sun.
Their discovery was not universally accepted when it was announced, for many researchers were suspicious. After all, if the element existed, why was it not found on Earth? Subsequently, however, it was. It was in 1895 that helium was discovered on Earth, emanating from a uranium-bearing element, called cleveite.
To conclude, one might think that mastery of the spectrum of the light emitted by the chemical elements in the Sun and stars would be what we need, but we’re just getting started. When we look at anything near us, if we can see it, it’s because light is reflected off the object and hits our eye. Typically, if we see a red object, it’s because it reflects red light and absorbs most of the other colors. However, some things are black, which means that they don’t reflect nearly as much light. And yet, even black things reflect some light.
Common Questions about Diffraction Gratings
German physicist Joseph von Fraunhofer built, what is called, a diffraction grating, a series of very closely spaced lines etched into a piece of glass.
Diffraction gratings are much more precise and are what is needed for astronomical studies. One of the most serious advantages of this technology is its reproducibility, meaning that researchers at different laboratories will get the same result.
Fraunhofer mapped out over 570 dark lines in the Sun’s spectrum; eleven that were especially dark, and the rest more minor in comparison. We still call these Fraunhofer lines.