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Fraunhofer, Josef von

lines sun’s spectrum fraunhofer’s

[ frown hohfer] (1787–1826) German physicist and optician: described atomic absorption bands in the solar spectrum.

Fraunhofer was apprenticed as a mirror-maker and lens-polisher in Munich, rising to become a director of his company in 1811. His miserable time as an apprentice was relieved when he was buried under the collapsed workshop and the Elector celebrated his survival with a gift of money which gave him independence. His interest in the theory of optics, and his scientific discoveries, led him eventually to become director of the Physics Museum of the Bavarian Academy of Sciences in 1823, but he died of tuberculosis 3 years later.

Fraunhofer’s main interest, and the motivation behind his experiments, was in producing a good quality achromatic lens. During the course of his investigations into the refractive properties of different glasses he used a prism and slit to provide a monochromatic source of light. In doing so, he noticed that the Sun’s spectrum was crossed by many dark lines, and he proceeded carefully to measure the wavelengths of almost 600 of them. Later, he used a diffraction grating to prove that the lines were not due to the glass of the prism but were inherent in the Sun’s light. These Fraunhofer lines were subsequently shown by to be due to atomic absorption in the Sun’s outer atmosphere, and tell us a great deal about its chemistry. Similar lines are observed for other stars and have been equally informative. Fraunhofer’s work was also important in establishing the spectroscope as a serious instrument, rather than merely a scientific curiosity.

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