Fraunhofer Lines
When a source (such as the sun) gives off light, that light can be dispersed into a rainbow spectrum by a prism or diffraction grating. If the light is pure white, the spectrum will contain all the colors of the spectrum. If, however, the light is not perfectly white, there will be areas within the spectrum where no color is present. These areas are distinguished by dark lines called Fraunhofer lines, after the German optician Joseph von Fraunhofer who first discovered them.
Fraunhofer earned a reputation as the premier glassmaker of his time. He was particularly famous for his optical instruments, such as the lenses used in telescopes; these lenses were more precise than the existing lens and were highly valued among European physicists. In order to test the quality of his lenses, Fraunhofer would callibrate them by passing through each lens the light from a candle and observing the spectrum produced. On one occasion, however, he chose to use the light from the sun to callibrate his lens. Upon examining the solar spectrum, Fraunhofer was surprised to find hundreds of dark lines crossing the colored band.
Though he did not know the cause of the dark lines, Fraunhofer knew he had made an important discovery. He was able to detect 574 different lines (primarily due to the high quality of his instruments), and he diagrammed these in detailed illustrations. Later, through painstaking calculation, he succeeded in estimating the wavelengths at which each line ocurred.
It is now known that Fraunhofer's lines are absorption lines and are caused by elements absorbing certain wavelengths of light. In the case of sunlight, white light is emitted by the sun' s core; however, the outer areas of the solar atmosphere contain many different elements, all of which will partially block the passage of light. Thus, when the sun's light reaches the Earth, it is missing a number of wavelengths; these missing wavelengths are characterized by dark lines in the spectrum.
The absorption lines were further studied in 1859 by the German physicist Gustav Kirchhoff. Through experimentation, Kirchhoff and his partner Robert Bunsen were able to distinguish which elements corresponded to which lines. Using this information, the two scientists identified what elements in the sun's atmosphere were causing Fraunhofer lines.
Bunsen and Kirchhoff soon discovered that elements, when heated, would emit light at the same wavelengths they absorbed, producing bright lines in the spectrum. Thus, by heating an unknown compound and examining its spectrum, scientists could indentify the elements that made up the compound. This formed the basis of the modern science of spectroscopy.
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