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.
In 1704, the renowned British physicist Sir Isaac Newton
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
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
The Fraunhofer dark lines have led to some interesting results. Sir John Lockyer
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
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,
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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. -
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. -
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. -
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. -
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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