n pl :  the dark lines in the spectrum of sunlight
 
Fraunhofer, Joseph von (1787-1826), German optician and physicist. Fraunhofer was a master theoretical optician as well as an expert maker of glass lenses and precision optical instruments. He effected the improvement of the homogeneity of optical glass and increased the size of the blanks free of imperfections, thus making possible the construction of large-diameter lenses. He sought to determine precisely the dispersion and index of refraction for different kinds of optical glass so that the construction of lenses could be based on optical theory and calculation. In 1814 he observed the effect of the refracting medium on light and found that the solar spectrum is crossed with many fine lines. He mapped over 570 lines that he observed between the red and violet ends of the spectrum. With these lines, now known as the Fraunhofer lines, as guides, he determined with unprecedented precision the optical constants of various kinds of glass.
 
 
 
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