Dark lines of the solar spectrum
A multitude of fine dark lines on the bright background of the solar spectrum. Turned into fixed reference marks, they made the spectrum a scale rather than a smear of paint.
The lines were noticed in 1814 by the German optician Joseph Fraunhofer while he was looking for a homogeneous light source to measure the dispersion of optical glass for the objectives of achromatic telescopes. He labelled the most distinct of them with the letters A to I, mapped about 574 lines and turned them into reference points by which the refractive index of a glass could be measured for a strictly defined colour. In 1821 Fraunhofer built a grating of 260 parallel wires, later ruled up to 3,200 lines per Paris inch with a diamond, and for the first time measured the wavelengths of individual lines.
Fraunhofer did not explain the nature of the lines themselves; that was done by the Germans Gustav Kirchhoff and Robert Bunsen in 1859 and 1860, and the lines then became a way of learning the composition of the Sun and the stars.
Why this date
exact: the observation of 1814 while measuring the refractive indices of optical glasses (DSB, Jenkins); publication Bestimmung des Brechungs- und Farbenzerstreuungs-Vermoegens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernroehre, Denkschriften der Koeniglichen Akademie der Wissenschaften zu Muenchen 5, 193-226; the volume covers 1814-1815 but appeared in 1817 (Teichmann, Stinner 2013). The axis value is 1814, the year of the first recorded observation. The diffraction grating of 260 wires and the work on diffraction: Denkschriften 8 (1821) and 1823. A few dark lines had been seen earlier by William Wollaston (1802); Fraunhofer found them independently, measured them and mapped them.
What it grew from
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Transparent blown glass
prerequisite for · Confidence: medium
The dark lines were found not in astronomy but in the glass trade: Fraunhofer was looking for a way to measure precisely the refraction and dispersion of kinds of optical glass for achromatic objectives, and the solar spectrum obtained through a prism of such glass turned out to be crossed by lines, which he then turned into reference marks for measurement.
Dispute: No direct assertion of this connection could be found in the historians: the continuity runs through the material and the craft, from Roman transparent blown glass to the optical glass of the early 19th century workshops. A separate caveat on the common claim that Fraunhofer melted his own glass: under the contract of 1809 he was sent to learn the secrets of melting from Pierre Louis Guinand and in 1809 to 1813 improved the homogeneity of optical glass together with him, but according to the factory records at Benediktbeuern he was responsible for all the downstream processing rather than for the melting itself; he took over the melting later.
Link sources
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026
In order to determine precisely the optical constants of glass, Fraunhofer in 1814 used the two bright yellow lines in flame spectra as a source of monochromatic light.
- Jenkins R. V. Verified 20 Sep 2026
Fraunhofer also sought to determine, with significantly greater precision than before, the dispersion and refractive index for different kinds of optical glass.
- Jenkins R. V. Verified 20 Sep 2026
Guinand and Fraunhofer, working together from 1809 to 1813, further improved the homogeneity of optical glass and increased the size of the striae-free blanks.
- Teichmann J., Stinner A. «From William Hyde Wollaston to Alexander von Humboldt: Star Spectra and Celestial Landscape», Annals of Science (2013) Verified 20 Sep 2026
Fraunhofer's interest lay in the construction of better achromatic refractor objectives.
- Jahn W., Fraunhofer-Gesellschaft, «Fraunhofer in Benediktbeuern: Glassworks and Workshop» Verified 20 Sep 2026
Fraunhofer was now responsible for the entire downstream processing, apart from the glass melting process.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026
What grew out of it
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Spectral analysis
prerequisite for · Confidence: high
Spectral analysis grew directly out of the explanation of Fraunhofer's lines: Kirchhoff showed that the dark D lines of the Sun stand exactly where the yellow lines of a sodium flame stand and arise from absorption, and his first communication of 1859 was titled precisely On the Fraunhofer lines.
Link sources
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (Encyclopedia.com) (1973) Verified 20 Sep 2026
It had long been known that the dark D lines, noticed in the solar spectrum by Fraunhofer (1814), coincided with the yellow lines emitted by flames containing sodium.
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (1973) Verified 20 Sep 2026
In particular, the interposition of a sodium flame of low temperature is sufficient to produce artificially the dark D lines in the spectrum of an intense light source which did not show them originally.
- «Kirchhoff, Gustav Robert», Encyclopedia of World Biography (1859) Verified 20 Sep 2026
the dark lines [Fraunhofer lines] of the solar spectrum which are not caused by the terrestrial atmosphere, arise from the presence in the glowing solar atmosphere of those substances which in a flame produce bright lines in the same position.
- Britannica, «Spectroscopy: Historical survey» (2024) Verified 20 Sep 2026
Joseph von Fraunhofer, repeating more carefully an earlier experiment by a British scientist, William Wollaston, had shown in 1814 that the spectrum of the Sun's electromagnetic radiation does not grade smoothly from one colour to the next but has many dark lines, indicating that light is missing at certain wavelengths because of absorption.
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026
Somewhat later he noted that some of these lines appeared to correspond to the bright doublet of lines in many flame spectra; yet he noted further that while the pattern observed for the sun and planets appeared identical, the patterns for the sun, Sirius, and other bright stars differed from one another.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.
- Rosenfeld L. «Kirchhoff, Gustav Robert», Dictionary of Scientific Biography (Encyclopedia.com) (1973) Verified 20 Sep 2026
Node sources
- Jenkins R. V. «Fraunhofer, Joseph», Dictionary of Scientific Biography (via Encyclopedia.com) Verified 20 Sep 2026
In order to determine precisely the optical constants of glass, Fraunhofer in 1814 used the two bright yellow lines in flame spectra as a source of monochromatic light.
- Jenkins R. V. Verified 20 Sep 2026
While conducting these tests, he observed the effect of the refracting medium on light, comparing the effect of light from flames with light from the sun, and found that the solar spectrum was crossed with many fine dark lines.
- Jenkins R. V. Verified 20 Sep 2026, quotation assembled from fragments
Designating the more distinct lines with capital letters (A,B,C,D,...,I), he mapped many of the 574 lines that he observed between B on the red end and H on the violet end of the spectrum.
- Jenkins R. V. Verified 20 Sep 2026
Extending his observations to diffraction resulting from a large number of slits, he constructed a grating with 260 parallel wires.
- Jenkins R. V. Verified 20 Sep 2026
Somewhat later he noted that some of these lines appeared to correspond to the bright doublet of lines in many flame spectra; yet he noted further that while the pattern observed for the sun and planets appeared identical, the patterns for the sun, Sirius, and other bright stars differed from one another.
- Teichmann J., Stinner A. «From William Hyde Wollaston to Alexander von Humboldt: Star Spectra and Celestial Landscape», Annals of Science (DOI 10.1080/00033790.2012.739709) (2013) Verified 20 Sep 2026
Fraunhofer's interest lay in the construction of better achromatic refractor objectives.
- Jahn W., Fraunhofer-Gesellschaft, «Fraunhofer in Benediktbeuern: Glassworks and Workshop» Verified 20 Sep 2026
He counted 574 lines of various thicknesses, which he drew and annexed to his Academy essay.
- High Altitude Observatory (UCAR), «Joseph von Fraunhofer (1787-1826)» Verified 20 Sep 2026
In 1821 Fraunhofer built the first diffraction grating, comprised of 260 close parallel wires.
- Britannica, «Spectroscopy: Historical survey» (2024) Verified 20 Sep 2026
Joseph von Fraunhofer, repeating more carefully an earlier experiment by a British scientist, William Wollaston, had shown in 1814 that the spectrum of the Sun's electromagnetic radiation does not grade smoothly from one colour to the next but has many dark lines, indicating that light is missing at certain wavelengths because of absorption.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.