Fraunhofer Lines

Fraunhofer Lines are dark lines crossing the solar spectrum or that of any other source of mixed light that is partially screened by an intervening gaseous medium. The simple spectrum of sodium gives two characteristic yellow lines close together. If white light containing that yellow be passed through a prism, it will be split up and will exhibit in a band the range of colour from red, through yellow, to the violet. But if the same light be surrounded with sodium vapour, that special yellow constituent of the mixed light will be absorbed in the gaseous envelope and will be absent from the spectrum, two black lines occupying exactly the same positions that the bright yellow held previously. Experiments of this kind have conclusively shown that the Fraunhofer lines indicate the substances present as gas in the intervening medium. In the case of the sun, this medium is the gaseous envelope surrounding the denser mass within. It is seen by the multitude of lines in its spectrum to contain many of the substances present on earth, such as sodium, potassium, calcium, iron, gold, silver, etc. The same principle is employed to compare the compositions of the various stars or other heavenly bodies that emit light. [Spectrum Analysis.]