Fraunhofer lines

In 1802, English chemist William Hyde Wollaston[2] was the first person to note the appearance of a number of dark features in the solar spectrum.

[3] In 1814, Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure their wavelengths.

[9] Some of the other observed features were instead identified as telluric lines originating from absorption by oxygen molecules in the Earth's atmosphere.

Absorption lines are narrow regions of decreased intensity in a spectrum, which are the result of photons being absorbed as light passes from the source to the detector.

The D1 and D2 lines form a pair known as the "sodium doublet", the centre wavelength of which (589.29 nm) is given the designation letter "D".

Wavelengths of the visual spectrum, 380 to about 740 nanometers (nm). [ 1 ] Dips in intensity are observed as dark lines at the wavelengths of the Fraunhofer lines, (e.g., the features G, F, b, E, B).
Solar spectrum with Fraunhofer lines as it appears visually.
Sun spectral irradiance measured with a calibrated spectrometer (Flame S-XR1-ES, Ocean Insight) mounted with a cosine corrector. The measurement was made on a sunny day on 13 June 2022 at noon from Hauterive, Switzerland (47°01′N   6°58′E). For photometry and colorimetry, standard measurements are usually carried out in the range 360–830 nm (highlighted area). Correlated color temperature (CCT): 5470 K.
Solar spectral irradiance measured with a calibrated optical spectrometer mounted with a cosine corrector. Some of the characteristic Fraunhofer lines and their corresponding elements are indicated for the extended visible spectrum (highlighted area in the graph).
For photometry and colorimetry, standard measurements are usually carried out in the range 360–830 nm. From these data and for this spectral range, the correlated color temperature (CCT) is 5470 K.
A demonstration of the 589 nm D 2 (left) and 590 nm D 1 (right) emission sodium D lines using a wick with salt water in a flame