Fraunhofer line

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The Fraunhofer lines or Fraunhofer lines are absorption lines in the spectrum of the sun . They arise through resonance absorption of the gases in the solar photosphere . The Fraunhofer lines allow conclusions to be drawn about the chemical composition and temperature of the gas atmosphere of the sun and stars.

The most important Fraunhofer lines in the visible part of the electromagnetic spectrum
Echelle grating spectrum of the sun with Fraunhofer lines

discovery

The English chemist William Hyde Wollaston was the first to observe dark lines in the solar spectrum in 1802. Independently of him, however, these were rediscovered in 1814 by the Munich optician Joseph von Fraunhofer , who then systematically studied them and determined their wavelengths through careful measurements. He recorded a total of over 570 lines, with the most prominent ones with the letters A to K (from long to short wavelengths; but without I and J). The less pronounced lines were given different letters.

Gustav Robert Kirchhoff and Robert Bunsen later discovered that each chemical element was associated with a specific number and arrangement of spectral lines. From this they concluded that the lines observed by Wollaston and Fraunhofer were due to the absorption properties of these elements in the upper layers of the sun and that these therefore also had to be present in the photosphere . However, some of the lines are also caused by the constituents of the earth's atmosphere.

The most important Fraunhofer lines
symbol element Wavelength in nm symbol element Wavelength in nm
y O 2 898.765 c Fe 495.761
Z O 2 822.696 F. H β 486.134
A. O 2 759,370 d Fe 466.814
B. O 2 686.719 e Fe 438,355
C. H α 656.281 G' H γ 434,047
a O 2 627,661 G Fe 430.790
D 1 N / A 589,594 G Approx 430.774
D 2 N / A 588.997 H H δ 410.175
D 3 or d Hey 587,562 H Ca + 396,847
e Ed 546,073 K Ca + 393,368
E 2 Fe 527.039 L. Fe 382,044
b 1 Mg 518,362 N Fe 358.121
b 2 Mg 517.270 P Ti + 336.112
b 3 Fe 516.891 T Fe 302.108
b 4 Fe 516.751 t Ni 299,444
b 4 Mg 516.733

application

Because of their known wavelengths, Fraunhofer lines are often used to determine the refractive index and dispersion of optical materials.

With the spectroscopic temperature determination, the surface temperature can be determined from the intensity distribution of the spectrum and with the help of the Boltzmann distribution . If, for example, the Balmer lines in the sun's spectrum can be observed as Fraunhofer lines , the temperature must be so high that some of the hydrogen atoms occupy the first excited state (n = 2). For example, in the sun with a surface temperature of 6000 K, every hundred millionth hydrogen atom is in the first excited state .

Evidence of the element helium was first obtained from a bright yellow spectral line at a wavelength of 587.49 nanometers in the spectrum of the sun's chromosphere . The French astronomer Jules Janssen made this observation in India during the total solar eclipse of August 18, 1868. When he made his discovery known, no one wanted to believe him at first, as a new element had never been found in space before the evidence on the Earth could be guided. On October 20 of the same year, the Englishman Norman Lockyer confirmed that the yellow line was indeed present in the solar spectrum and concluded that it was caused by a previously unknown element.

Others

The Fraunhofer C-, F-, G'- and h-lines correspond with the alpha-, beta-, gamma- and delta-lines of the Balmer series of a hydrogen atom.

The lines A, B, a, Y and Z are not of solar but of terrestrial origin, that is, they are created by absorption in the earth's atmosphere .

Individual evidence

  1. ↑ Joseph Fraunhofer: Determination of the refraction and color dispersal ability of different types of glass, in relation to the perfecting of achromatic telescope . In: Annals of Physics . tape 56 , no. 7 , 1817, pp. 264-313 , doi : 10.1002 / andp.18170560706 .
  2. ^ Francis A. Jenkins, Harvey E. White: Fundamentals of Optics. 4th edition. McGraw-Hill, 1981, ISBN 0-07-256191-2 , p. 18.
  3. ↑ Dietrich Lemke: Helium solar element on the Big Bang Part 1: The discovery of helium . In: Prof. Dr. Matthias Bartelmann (ed.): Stars and space . No. 1 | 2020 . Spectrum of Science, January 2020, ISSN  0039-1263 , p. 41 ff .

Web links

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