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...