German physicist Joseph von Fraun-hofer indepen-dently rediscovers and characterizes spectral dark lines.

John Herschel pos-its that an atom’s capacity to absorb a color—and its inabil-ity to emit it when excited—account for dark and bright lines in its spectra.

English scientist William Hyde Wol-laston observes black lines in the solar spectrum; he attributes them to instrument artifacts.

Barry R. Masters

A Brief History of

German physicist German physicist Joseph von Fraun-Joseph von Fraun-hofer indepen-hofer indepen-dently rediscovers dently rediscovers and characterizes and characterizes spectral dark lines.spectral dark lines.

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7Isaac Newton dem-onstrates that white light is composed of many colors and they can be dis-persed by a prism due to their different indices of refraction.

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Scottish scientist William Swan cor-rectly surmises that two dark lines were due to sodium in the light source or its vicinity.

Henry Augustus Rowland, a U.S. physicist, produces new standard sets of spectral lines with his spectroscope.

William Ram-say discovers helium in ura-nium minerals.

William Ram-William Ram-say discovers say discovers helium in ura-helium in ura-nium minerals.nium minerals.

From Newton’s fi rst insight into the composition of sunlight to the discovery of helium on the sun two centuries later, scientists’ work on the measurement and analysis of light has led to important discoveries that have greatly expanded our knowledge of physics, chemistry and astronomy.

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French astronomer Pierre Janssen and Brit-ish astronomer Joseph Norman Lockyer inde-pendently observe an unknown bright yellow spectral line—the fi rst evidence of helium.

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Kirchhoff works with Robert Bunsen to develop prism spectroscopes and the technique of spectral analysis.

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Gustav Robert Kirch-hoff proposes his law of the equivalence of emission and absorp-tion spectra under identical conditions.

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William Huggins William Huggins and W.A. Miller and W.A. Miller present to the present to the Royal Society the Royal Society the results of their results of their investigations into investigations into the chemical con-the chemical con-stitution of stars. stitution of stars.

Images from Wikimedia Commons

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pectral analysis is important in many branches of chemistry, physics and the life sciences. Its foundation lies in the phenomenon that different forms of matter produce characteristic absorption and emission spectra that can be used to identify

their atomic and molecular structure. The history of mod-ern physics—and particularly the development of quantum mechanics—was motivated by studies of the wavelength dependence of black body radiation and the precise spectro-scopic measurements of atoms and molecules, including their light absorption and emission wavelengths, polarization, selec-tion rules and intensities.

The beginnings of spectroscopy date back more than three centuries to the time when Sir Isaac Newton first elucidated the color composition of sunlight. Since then, scientists from all over the world have worked to characterize and measure light—from William Hyde Wollaston’s first observation of seven dark lines in the solar spectrum in 1802 to Fraunhofer’s systematic investigation of those lines in 1814 to the develop-ment of prism spectroscopes and experimental spectral analysis by Kirchhoff and Bunsen in 1860. This work in turn paved the way for the discovery of the element helium on the sun by two independent astronomers in 1868.

This article traces the history of spectral analysis and astrospectroscopy and sheds insight into how scientists used spectroscopes and telescopes to make precision measurements and realize new scientific discoveries—some of which were out of this world! As we will see, the story of spectroscopy is a tes-tament to the fecundity of interdisciplinary studies: Chemists, physicists and astronomers all made important contributions to the field and the development of precision instrumentation and measurements.

Issac Newton’s experiments with prisms, lenses and sunlightAt the time of Isaac Newton (1643-1727), scientists thought that white light was colorless and that individual colors were formed within a prism. Newton used prisms to experiment with the spectral decomposition of sunlight and debunk this common belief. For example, when he separated red light from a prism and passed the red light through a second prism, he found that the color remained unchanged—thus showing that color was not formed within the prism but rather was an inherent property of white light. Subsequently, with a lens and another prism, Newton demonstrated that the multicolored dispersed light could be recombined into white light.

Of seminal importance were his studies into chromatic aberration, which occurs when each color within white light is focused by a lens at a different axial position. This type of aberration can be a serious problem in refracting telescopes (in which image formation occurs via lenses). When white light passes through air and glass, all the colors do not focus at the same point because each color or frequency has a unique

refractive index. The result is that objects may appear to be surrounded by fuzzy, rainbow-colored haloes.

In his 1704 book Optics, Newton proposed a practical solution—the use of two lenses of different refractive indices to correct chromatic aberration. However, because he incor-rectly believed that the incident and emergent light must be parallel, Newton ultimately abandoned his pursuit of refract-ing telescopes. Instead, he developed telescopes with reflecting lenses and mirrors that are devoid of chromatic aberration.

Fraunhofer’s calibration with black solar absorption linesJoseph von Fraunhofer (1878-1826) was a German physicist known for his remarkable contributions to the manufacture of optical glass as well as his research on the solar spectrum. In 1817, he published an essay Determination of the Refractive and Dispersive Indices of Differing Types of Glass in Relation to the Perfection of Achromatic Telescopes. Fraunhofer, who never attended a university, learned optics as an apprentice.

Since the time of Newton, the solar spectrum appeared to be continuous. The lack of specific colors or wavelengths that could serve as references hindered any attempts to measure the refraction of glass at each wavelength—until Fraunhofer came along. He made such precise assessment possible when he invented a spectroscope in 1814. The light from the source passed through slits and was incident upon a flint glass prism. Fraunhofer detected the dispersed light from the first prism by eye in another spectroscope several meters away. In the second spectroscope, the detecting prism was precisely rotated about an angle on an axis perpendicular to the line between the two prisms. Finally, Fraunhofer observed the light with a telescope. Because the observation prism and telescope were mounted on a precise rotation stage that was accurate to one arcsecond,

S

A replica of Isaac Newton’s second

reflecting tele-scope of 1672.

Wikimedia Commons

Fraunhofer could calculate the refractive index of the test prism for each wavelength to an accuracy of six decimal places.

When he turned his spectroscope to other light sources, he made an amazing discovery. Over the continuous solar spec-trum, Fraunhofer observed the existence of numerous black lines of varying thickness. He began to make a series of precise measurements of the wavelengths of these lines, and he used them to mark specifi c wavelengths in the solar spectrum.

In this way, he developed a method of measuring the refractive index of glasses at a set of precisely determined and reproducible wavelengths—which allowed him to produce opti-cal glasses with a high reproducibility and precision. Eventually, Fraunhofer measured the wavelengths and characterized the intensities for 574 lines in the solar spectrum. He used the lines to delineate the separations of various colors and to measure the refractive indices of glasses in order to produce his world famous achromatic lenses. His glass laboratory was housed in an old Benedictine monastery in Bavaria.

(Interestingly, Fraunhofer wasn’t the fi rst to discover these demarcations. Th e lines were in fact fi rst noticed in 1802 by the English chemist and physicist William Hyde Wollaston, who attributed them to instrument artifacts. It wasn’t until 1824 that Fraunhofer learned of Wollaston’s prior discovery from John Herschel, the notable English mathematician, astronomer, inventor and chemist.)

Fraunhofer observed a doublet of two close yellow lines in the emission spectrum of a lamp, and the measured wavelengths of this doublet (which he called “D lines”) exactly corresponded to two dark lines in the solar spectrum. Later, in 1830, the ori-gin of these dark lines became an active topic of debate among David Brewster, George Biddell Airy and John Herschel.

After Fraunhofer made his discovery, some scientists specu-lated that the line spectra were related to the structure of atoms. For example, in 1827, John Herschel posited that an atom’s capacity to absorb a specifi c color—and its inability to emit the same color when thermally excited—accounted for the dark and bright lines in its spectra. In 1852, George Gabriel Stokes off ered similar explanations. In 1856, William Swan correctly surmised that the two dark lines were due to the presence of sodium in the light source or in its vicinity.

Gustav Robert Kirchhoff, Robert Bunsen and William HugginsTh e stimulus for the experimental study of spectra came in 1859 when Gustav Robert Kirchhoff proposed his law of the equivalence of emission and absorption spectra under identical

Kirchhoff, the Bunsen burner and spectroscopyGustav Robert Kirchhoff was born on March 12, 1824, in Köningsberg, Prussia, and died on October 17, 1887, in Berlin, Germany. A student of Gauss, he was called to Heidelberg as a profes-sor of physics in 1854. Although he is perhaps most famous for his work on the theory of electrical circuits (Kirchhoff’s laws), he also studied black body radiation and spectral analysis. In fact, he collaborated with the chem-ist Robert Bunsen (1811-1899) to develop chemical analysis based on the observation of spectra. Using a spectroscope, he and Bunsen discovered cesium (1860) and rubidium (1861).

Bunsen and his assis-tant Peter Desaga—an instrument-maker at the University of Heidelberg— invented the Bunsen burner, a source of very high fl ame temperatures that was an essential com-ponent of the Kirchhoff-Bunsen spectroscope. In order to observe low intensity lines, Bunsen and Desaga needed to produce a gas fl ame that met two requirements: (1) a very high temperature and (2) low luminescence. The Bunsen burner achieved this by premixing the air and the gas before the mixture underwent combustion.

Kirchhoff investigated the composition of light from the sun. He was the fi rst to offer a seminal physical explanation for the dark lines in the sun’s spectrum; he posited that they were caused by the absorption of specifi c wavelengths as light passed through a gas. Kirchhoff’s three laws of spectros-copy are: (1) A hot solid material emits light with a continuous spectrum; (2) A hot gas emits light with spectral lines at discrete wavelengths; (3) A hot solid material that is surrounded by a cool gas emits light with a continuous spectrum that has gaps (black lines) at discrete wavelengths that depend on the type of atoms in the gas.

Kirchoff

Bunsen

In 1860, Kirchhoff worked with Robert Bunsen to develop prism spectroscopes and the experimental technique of spectral analysis in which they demonstrated that the spectrum of the luminescence of materials could be used to characterize those materials.

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conditions. Kirchhoff based this law on thermodynamic prin-ciples. He eventually extended it to include the use a prism to disperse the light emitted from fl ames.

In 1860, Kirchhoff worked with Robert Bunsen to develop prism spectroscopes and the experimental technique of spectral analysis in which they demonstrated that the spectrum (inten-sity versus wavelength or frequency) of the luminescence of materials could be used to characterize those materials. In their fi rst spectroscope, the fl ame from a Bunsen burner was the light source; the light then illuminated a narrow slit, passed through a collimator lens, and became incident on a prism that could be precisely rotated. Th e prism dispersed the light, which was observed with a telescope that contained an objective lens and an ocular.

In the next decade, Kirchhoff , Bunsen and others con-structed various spectroscopes, and the spectroscopy of various substances became an active area of research. In 1860 and 1861, Kirchhoff and Bunsen published two extensive papers in the journal Annalen der Physik und Chemie under the title “Chemical Analysis through Spectral Observations.” Th ey used this massive collection of spectroscopic data to support their claim that each element produces a distinct set of spectral lines. Th ey also postulated that the dark solar lines were due to elements on the sun with distinct absorption lines. With their spectroscope, they characterized the spectra of sodium, lithium, potassium, strontium, calcium and barium.

Before long, William Huggins (1824-1910), a pioneer in astrophotography, shifted his attention to the chemical con-stitution of the stars. He attached a spectroscope to a stellar telescope in order to examine stellar spectral lines. Th at was the start of astrospectroscopy. Huggins and his colleague W.A. Miller presented their initial results to the Royal Society in 1863; their note was entitled “Lines of some of the fi xed stars.” Huggins’ seminal contribution was his use of gelatin dry pho-tographic plates, which permitted long integration times for astronomical images and spectral lines. Th is advance was criti-cal for enabling the detection of extremely weak spectral lines.

Precision measurements are needed in order to advance the theory of the origin of spectral lines. In 1868, S.J. Angstrom published his compendium of 1,000 solar lines, which he obtained with a spectroscope that used diff raction gratings. His tables replaced the arbitrary units in Kirchhoff ’s tables of spectra data, and they remained the standard until Henry Augustus Rowland, a U.S. physicist, produced a new set with his spectroscope, which contained higher-resolution diff raction gratings: the Photographic Map of the Normal Solar Spectrum (1888) and the Table of Solar Wave-Lengths (1898).

Janssen and Lockyer independently discover heliumWhen the French astronomer Pierre Janssen (1824-1907) was watching an eclipse of the sun in Guntur, India, he made a seminal observation. Janssen was examining the sunlight

Spectral lines and dispersing elements A spectral line is not typically a single frequency; rather, it extends over a range of frequencies (nonzero line width). Vari-ous physical processes are responsible for both spectral line broadening and shifts in frequency (for example, natural line broadening, Doppler broadening and pressure broadening).

A continuous spectrum is produced by heating a black body. The spectrum is recorded as a continuous range of colors and is analyzed by a plot of intensity (corrected for detector sensitivity) versus wavelength or frequency.

An absorption spectrum is similar to the continuous spectrum (from the source of illumination) but it contains black absorption lines across the spectrum due to the presence of matter between the source of illumination (a lamp, a Bunsen burner fl ame, a star) and the detector of the spectroscope. The matter undergoes electronic transitions and absorbs specifi c frequencies of the incident light.

Emission lines are composed of a narrow range of fre-quencies or wavelengths that are observed against a dark background. They are due to the emission of light from an atomic or molecular electronic transition in which the source of illumination is not detected. An example is the Balmer series of emission lines that correspond to electronic transi-tions in hydrogen. Due to the high abundance of hydrogen in the universe, these emission lines are often observed and are relatively intense compared to lines from other elements.

Dispersing elements are what scientists use to disperse and separate light into its component wavelengths. The fi rst dis-persing elements were prisms (transparent optical elements with fl at polished surfaces that refract light) constructed from materials with a high index of refraction. Prisms were used in the spectroscopes of Kirchhoff and Bunsen.

In the Kirchhoff/Bunsen spectroscope, one face of the prism is fi lled with white light that is made parallel with a lens; the prism disperses and resolves the light, which is focused by another lens onto the detector. In a line spectrum, the image of the entrance slit of the spectroscope is formed at a specifi c wavelength; the image is a diffraction pattern of the slit. If the wavelengths λ and λ + dλ, are just separated, then dλ is the resolution obtained in the spectroscope. The physi-cal principle behind a dispersive prism is that the refractive index of the prism depends on frequency; thus it can disperse white light into its spectral colors.

Today, prisms are typically superseded by diffraction gratings as dispersing elements. A refl ection diffraction grating is a series of parallel grooves cut on a hard glass or metallic surface. The space between adjacent grooves may be or the order of 1 μm, and the surface is usually coated with aluminum so the grating also acts as a mirror. The dif-fraction grating can be either planar or concave; the latter can also focus as well as disperse the incident light.

For light incident at 90° to the surface of the grating, the diffraction is given by mλ = d(sin i + sin θ), where i and θ are the angles of incidence and refl ection, measured from the normal to the surface, d is the groove spacing, λ is the wavelength, and m (0, 1, 2, 3…) is the order of diffraction. The resolving power of a diffraction grating is proportional to the total number of grooves. At higher orders of diffraction, the problem of spectral overlap of adjacent diffraction orders is increased.

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[ References and Resources ]

>> R. Bunsen. Gesammelte Abhandlungen. 3 Bände, Leipzig, Engelmann (1904).

>> G. Kirchhoff. “Über das Verhältnis zwischen dem Emissionsver-mögen und dem Absorptionsvermögen der Körper für Wärme und Licht.” Annalen der Physik 109, 275-301 (1860).

>> J.N. Lockyer. The Spectroscope and Its Applications. London, MacMillan and Co. (1873). [available on Google Books]

>> J.N. Lockyer. Studies in Spectrum Analysis. London: Kegan Paul, Trench, Trübner & Co. Ltd. (1904).

>> Sir I. Newton. Opticks, Fourth Edition, 1730 [first published 1704], New York, Dover Publications (1952).

>> J. Agassi. The Kirchhoff-Planck Radiation Law, Science 156, 30-7 (1967).

>> W. McGucken. Ninteenth-Century Spectroscopy, Development of the Understanding of Spectra 1802-1897. Baltimore, Johns Hop-kins Press (1969).

>> A.J. Meadows. Science and Controversy: A Biography of Sir Nor-man Lockyer. Cambridge, Mass., MIT Press (1972).

>> H. Kangro. Gustav Kirchhoff: Untersuchungen über das Sonnen-spectrum und die Spectren der chemischen Elemente und weitere ergänzebde Arbeiten aus den Jahren 1859-1862. Osanabrück, Zeller (1972).

>> J.A. Bennett. The Celebrated Phaenomena of Colours: The Early History of the Spectroscope. Cambridge, U.K., Whipple Museum of the History of Science (1984).

>> J.B. Hearnshaw. The Measurement of Starlight, Two Centuries of Astronomical Photometry, New York: Cambridge University Press (1996).

>> M.W. Jackson. Spectrum of Belief, Joseph von Fraunhofer and the Craft of Precision Optics. Cambridge, Mass., The MIT Press (2000).

>> K. Hentschel. Mapping the Spectrum, Techniques of Visual Representation in Research and Teaching, Oxford, U.K., Oxford University Press (2002).

>> H. Kragh. “The Solar Element: A Reconsideration of Helium’s Early History,” Annals of Science 66(2), 157–82 (2009).

with a telescope and spectroscope on August 18, 1868, when he noticed an unknown bright yellow spectral line signature—it was the first evidence of helium. It appeared at a wavelength of 587.49 nm in the spectrum of the chromo-sphere of the sun. Janssen is also credited with demonstrating the gaseous nature of the red solar prominences that were observed during solar eclipses.

Janssen had studied mathematics and physics, but he was mainly involved in scientific expeditions to Asia, Africa, Europe, and the Pacific Islands from 1860 to 1905. On these trips, he studied eclipses, and, that year, he had discovered a technique to observe solar prominences that occurred in the absence of an eclipse.

Tests were soon carried out at the College of Chemistry in London to reproduce the line that Janssen had observed. Although its existence was confirmed, no one knew its source. Janssen postulated that it was an element that had not yet been discovered on our planet—a conjecture that attracted much ridicule since no elements had been detected on the sun that were not known to exist on earth.

Meanwhile, on October 20 of that same year, another scientist made the same independent discovery. In a remark-able leap of intuition, the British astronomer Joseph Norman Lockyer (1836-1920) combined an astronomical telescope, which he built himself, with a prism spectroscope. Using this setup, Lockyer discovered the same yellow line that Janssen had observed. He named it the D3 Fraunhofer line because it was near the known D1 and D2 lines of sodium. He also concluded that its origin was an unknown chemical element that had not yet been found on earth.

Lockyer collaborated with the English chemist Edward Frankland, who had previously investigated how the spectrum of elements related to their physical state, on the identification of helium. They named the element helium, after the Greek word for the sun (helios).

(Lockyer is also well-known for establishing the journal Nature in 1896. One of his primary objectives in doing so was to facilitate the communication of ideas between disparate scientific disciplines; this goal seems so relevant today. For the next 50 years, he remained the journal’s editor until just before his death in 1920.)

Some 25 years after Janssen and Lockyer discovered helium on the sun, a Scottish chemist found the first evidence of it on our home planet. In 1895, William Ramsay discovered helium in uranium minerals. In another example of independent discovery, two Swedish chemists—Per Teodor Cleve and Nils Abraham Langlet—also discovered helium in a mineral called cleveite at around the same time.

Concluding thoughtsIn 1835, the French positivist philosopher Auguste Comte wrote that people would never understand the chemical constitution of the stars. Thankfully, through spectroscopy, science has proven Comte wrong. In 1904, after Pierre Janssen and J. Norman Lockyer independently discovered helium on the sun, Lockyer wrote at the beginning of his book Studies In Spectrum Analysis: “The work of the true man [woman] of Sci-ence is a perpetual striving after a better and closer knowledge of the planet on which his lot is cast, and of the universe in the vastness of which that planet is lost.” t

Barry R. Masters (brmail2001@yahoo.com), OSA Fellow, SPIE Fellow, is with the department of biological engineering, Massachusetts Institute of Technology, Cambridge, Mass., U.S.A.

Janssen was examining sunlight with a telescope and spectroscope on August 18, 1868, when he noticed an unknown bright yellow spectral line signature—it was the first evidence of helium.

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