Fraunhofer Invents the Spectroscope Summary

  • Last updated on November 10, 2022

Joseph von Fraunhofer discovered that sunlight, when passed through a glass prism or a grating, produced a spectrum of colors that contained numerous dark lines. Later investigators showed that these dark lines were produced by specific chemical elements.

Summary of Event

In 1704, the renowned British physicist Sir Isaac Newton Newton, Sir Isaac published Optiks, which described his wide-ranging investigations into the properties of light. He measured the angular dispersion of sunlight into a spectrum of colors by using a triangular glass prism. He also gave a mathematical Mathematics;and refraction[Refraction] explanation for the creation of the rainbow due to refraction of sunlight by water droplets in the atmosphere. Spectroscope Fraunhofer, Joseph von Optics Glass;prisms [kw]Fraunhofer Invents the Spectroscope (1814) [kw]Invents the Spectroscope, Fraunhofer (1814) [kw]Spectroscope, Fraunhofer Invents the (1814) Spectroscope Fraunhofer, Joseph von Optics Glass;prisms [g]Germany;1814: Fraunhofer Invents the Spectroscope[0660] [c]Inventions;1814: Fraunhofer Invents the Spectroscope[0660] [c]Physics;1814: Fraunhofer Invents the Spectroscope[0660] [c]Chemistry;1814: Fraunhofer Invents the Spectroscope[0660] [c]Astronomy;1814: Fraunhofer Invents the Spectroscope[0660] [c]Photography;1814: Fraunhofer Invents the Spectroscope[0660] [c]Mathematics;1814: Fraunhofer Invents the Spectroscope[0660] Kirchhoff, Gustav Robert Bunsen, Robert Wollaston, William Hyde Young, Thomas

About one hundred years later, British scientist William Hyde Wollaston saw something in the spectrum of sunlight that neither Newton nor anyone else had noted before. Wollaston’s experiment involved sunlight moving through a narrow slit into a dark room, where the sunlight struck a prism. The resulting spectrum was observed from ten feet away. At that distance the colors from red to violet were greatly spread out. Wollaston noticed that the continuous spectrum of the sun had some narrow, dark lines in it. Whereas an ordinary light source viewed through a prism emits a truly continuous spectrum of colors, sunlight appears to have some missing wavelengths. He reported finding seven dark lines but had no explanation for what caused them.

In 1814, twelve years after Wollaston’s discovery, Joseph von Fraunhofer independently discovered through his own experimentation the dark lines in the spectrum of the sun. He devised a special apparatus, the spectroscope, that enabled him to catalog more than five hundred dark lines, now called Fraunhofer lines. The spectroscope had a lens that could be pointed at the sun or any other source of light, followed by a narrow slit. The incoming light beam struck a prism made of flint glass that produced a relatively large angular separation of colors. The spectrum was viewed through an eyepiece attached to a platform that could be rotated, allowing the angle of view to be measured with high precision. The most prominent dark lines were given letter names. Fraunhofer noted that the “D” line in the solar spectrum exactly matched the angle of sodium light that had been observed previously. However, he was not able to determine the significance of this observation.

The British scientist Thomas Young Young, Thomas earlier had shown that a light beam, when passed through two slits that are very close together, produced an interference pattern of bright and dark images on a screen. He explained this pattern using the wave theory of light: When two waves are in step, their amplitudes will add to produce a brightness, but when they are half a wavelength out of step, their amplitudes will cancel to produce darkness. Young developed a mathematical formula that used the distance between the two slits and the angles of maximum brightness to calculate the wavelength of the light. Fraunhofer improved on Young’s double slit by making a grating, consisting of a large number of closely spaced parallel slits. He wound a thin metal wire back and forth between two threaded screws. By advancing from one thread to the next one, he obtained a closely spaced mesh of wires.

Fraunhofer replaced the prism in his spectroscope with such a grating. The angular separation of colors, or dispersion, was not much better than his result had been with the prism. To improve his observations further, he needed to make the slits in the grating even closer together. As part owner of a glassworks company, Fraunhofer had access to a machine shop. A new grating was made by scribing hundreds of evenly spaced parallel lines on a piece of glass. Light came through the spaces to form a spectrum with high dispersion. With this device, he was able to measure the wavelength of yellow light from a sodium flame with a precision that agrees within 1 percent of the modern accepted value.

Fraunhofer was not an academic scientist. He was a craftsman skilled in making glass lenses for optical instruments. He used the solar dark lines as fixed calibration points to measure how the index of refraction of glass varied throughout the spectrum. He learned how to combine lenses of different glass composition into an achromatic system that gave the sharpest possible images. He became famous throughout Europe as the premier supplier of lenses for large telescopes.

Gustav Robert Kirchhoff, Kirchhoff, Gustav Robert a physicist, and Robert Bunsen, Bunsen, Robert the chemist of Bunsen burner fame, were colleagues at the University of Heidelberg in Germany. During the 1850’s, they were studying the spectra of flames that contained various chemicals such as sodium, potassium, and copper salts. Using a grating in a spectroscope, they observed that each element had a unique spectrum of bright lines. These emission spectra provided them with an unambiguous identification, like a fingerprint, for each element. Kirchhoff and Bunsen were aware of Fraunhofer’s work thirty-five years earlier on dark lines in the spectrum of sunlight. In trying to understand these lines, Kirchhoff set up a crucial experiment. Using a laboratory lamp, he showed that it had a true continuous spectrum with no dark lines. Then he placed a sodium flame between the lamp and the grating. This time the continuous spectrum had a dark line in the yellow region, just at the known wavelength of sodium. Evidently, sodium vapor was absorbing its particular wavelength out of the continuous spectrum.

Kirchhoff Kirchhoff, Gustav Robert and Bunsen Bunsen, Robert proposed the idea that atoms have an absorption spectrum that matches their emission spectrum. They were able to show that three prominent Fraunhofer dark lines in the solar spectrum exactly matched the emission wavelengths of potassium. They concluded that light from the surface of the sun was being absorbed at fixed wavelengths by sodium, potassium, and other atoms in the sun’s outer atmosphere.

Significance

The Fraunhofer dark lines have led to some interesting results. Sir John Lockyer Lockyer, Sir Joseph Norman , a British astronomer, speculated in 1868 that a prominent dark line in the solar spectrum, which did not match any element known on Earth, might be caused by a new element found only on the sun. He named it “helium,” Helium after the Greek word for the sun. Some thirty years later, helium Helium;discovery of gas eventually was found on Earth in deep mineshafts. Mining;and helium gas[Helium gas] Helium has become a valuable resource for various technological applications, as well as for lighter-than-air balloons.

Fraunhofer dark lines are found not only in the spectrum of the sun but also in all stars. Astronomers can use telescopes to focus on one star at a time and can record its spectrum on photographic Photography;and astronomy[Astronomy] Astronomy;and photography[Photography] film. Film;and astronomy[Astronomy] In some cases, the Fraunhofer lines show shifts toward longer wavelengths, that is, toward the red end of the spectrum. “Red shift” occur when stars are moving away from the earth at high speeds. This phenomenon is like the drop in frequency that one hears when an ambulance with a siren is traveling away from the listener. The red shift in the Fraunhofer lines from distant stars is the primary evidence for an expanding universe.

Spectroscopy has been extended to other parts of the electromagnetic spectrum as new instrumentation became available. For example, infrared spectra are the primary means to obtain information about the structure of molecules. Gamma ray spectroscopy has become a highly developed method of analysis that can detect impurities in materials as small as a few parts per billion. Fraunhofer’s spectroscope was the starting point for many practical applications in analytical chemistry, Chemistry;and spectroscopy[Spectroscopy] astronomy, Astronomy;and spectroscopy[Spectroscopy] medical research, and other technologies.

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Connes, Pierre. “How Light Is Analyzed.” Scientific American (September, 1968): 72-82. Gives line drawings and photographs that show how the prism and grating are used for optical spectroscopy. Includes a section on modern instrumentation.
  • citation-type="booksimple"

    xlink:type="simple">Jackson, Myles. Spectrum of Belief: Joseph Fraunhofer and the Craft of Precision Optics. Cambridge, Mass.: MIT Press, 2000. A thoroughly researched book on the life and work of Fraunhofer, containing an appendix with explanatory notes and an extensive bibliography.
  • citation-type="booksimple"

    xlink:type="simple">Jenkins, Reese V. “Fraunhofer, Joseph.” In Dictionary of Scientific Biography. Vol. 5. New York: Charles Scribner’s Sons, 1981. An authoritative compilation of biographies, with several pages devoted to Fraunhofer’s career.
  • citation-type="booksimple"

    xlink:type="simple">Taylor, Lloyd W. Physics, the Pioneer Science. Cambridge, Mass.: Riverside Press, 1941. An excellent introductory physics textbook with much historical information on scientists. Out of print but likely available in academic libraries.
  • citation-type="booksimple"

    xlink:type="simple">Walker, James S. Physics. 2d ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2004. A college-level textbook explaining interference of light waves, including Thomas Young’s use of the double slit and Fraunhofer’s grating.

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