Fraunhofer lines : The Rainbow lines

 

225th Birthday Joseph von Fraunhofer

Dater of Issue : 2 January 2012

 

German Post issued a postage stamp on 2 January 2012 to commemorate 225th Birthday Joseph von Fraunhofer, The stamp features  the spectral lines called Fraunhofer lines. It is a beautiful   stamp showing Rainbow colors and one of the best stamps for my Rainbow Collection !!

The colors symbolize the solar spectrum with Fraunhofer lines.

Fraunhofer lines

Solar spectrum with Fraunhofer lines as it appears visually.

In physics (optics) , the Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

The Fraunhofer lines are typical spectral absorption lines. These dark lines are produced whenever a cold gas is between a broad spectrum photon source and the detector. In this case a decrease in the intensity of light in the frequency of the incident photon is seen as the photons are absorbed, then re-emitted in random directions, which are mostly in directions different from the original one. This results in an absorption line, since the narrow frequency band of light initially traveling toward the detector, has been effectively scattered in other directions. Absorption lines are produced even during reflection from an illuminated cold gas, since after reflection there is still the opportunity for a selective absorption (and re-scatter) between the point of reflection and the detector. By contrast, if the detector sees photons emitted directly from a glowing gas, then the detector often sees photons emitted in a narrow frequency range by quantum emission processes in atoms in the hot gas, resulting in an emission line. In the Sun, Fraunhofer lines are seen from gas in the outer regions of the Sun, which are too cold to directly produce emission lines of the elements they represent.