Fraunhofer lines

Fraunhofer lines are dark features at specific wavelengths in the Sun’s spectrum, produced mainly when atoms and ions in cooler solar gas absorb light.

Atomic absorption spectrum: A pattern of wavelengths absorbed when light passes through atoms or ions. Fraunhofer lines are the solar example of this general spectral pattern.
Joseph von Fraunhofer: A German optician and physicist who measured dark lines in the solar spectrum in the early nineteenth century. His careful measurements established the lines’ positions and the naming convention still used.
Solar composition: The elemental and isotopic makeup of the Sun. Matching solar line wavelengths to laboratory spectra identifies elements in the Sun.
Kirchhoff's laws of spectroscopy: Rules relating continuous, emission, and absorption spectra to the conditions of a light source and intervening gas. The solar lines helped connect dark spectral features to cooler absorbing gas.
Bohr model: A model of the hydrogen atom in which electrons occupy discrete energy levels. Quantized energy levels help explain why atoms absorb only particular wavelengths.
Optical spectroscopy: The study of how matter emits, absorbs, or scatters light across optical wavelengths. Fraunhofer’s work helped establish spectroscopy as a tool for studying matter.
Helium: A chemical element with atomic number two, first identified in the solar spectrum before its discovery on Earth. A yellow solar spectral line revealed helium in 1868.
Spectroscopy: The study of interactions between matter and electromagnetic radiation as a function of wavelength or frequency. Fraunhofer lines helped establish spectra as evidence about matter beyond direct sampling.
Doppler effect: A change in observed frequency caused by relative motion between a source and an observer. Solar motion shifts line wavelengths, letting astronomers measure motion from the spectrum.
Fraunhofer diffraction: A far-field approximation describing diffraction when source and observation distances are large. It shares Fraunhofer’s name but describes wave diffraction, not solar absorption lines.

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