Fraunhofer lines, (C) Wikipedia

Spectral absorption lines of the spectrum of the sun, named after the German physicist Joseph von Fraunhofer (1787–1826)
Such Lines are produced whenever a cold gas is between a broad spectrum photon source and the detector.
Photons are absorbed and then re-emitted in random new directions. Therefore the intensity in the original direction is lower and a dark line appears in the spectrum.

[table]
wavelength in nm, Fraunhopfer line, Lightsource, color
365.0146,i,Hg,UV
404.6561,h,Hg,violet
435.8343, g, Hg, blue
479.9914, F’, Cd, blue
486.1327, F, H, blue
546.0740, e, Hg, green
587.5618, d, He, yellow
589.2938, D, Na, yellow
643.8469, C’, Cd, red
656.2725, C, H, red
706.5188, r, He, red
768.2, A’, K, red
852.11, s, Cs, IR
1013.98, t, Hg, IR
[/table]
See also Abbe-number, dispersion.