Joseph von Fraunhofer

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Joseph von Fraunhofer bigraphy, stories - German optical physicist

Joseph von Fraunhofer : biography

6 March 1787 – 7 June 1826

Joseph Fraunhofer, ennobled in 1824 as Ritter von Fraunhofer (6 March 1787 – 7 June 1826) was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun’s spectrum, and for making excellent optical glass and achromatic telescope objectives.

Notes

Telescopes and optical instruments

Fraunhofer produced various optical instruments including microscopes for his firm. This included the Fraunhofer Dorpat Refractor used by Struve (delivered 1824), and the Bessel Heliometer (delivered posthumously), which were both used to collect data for stellar parallax. The firm’s successor, Merz und Mahler, made a telescope for the New Berlin Observatory, which confirmed the existence of the major planet Neptune. Possibly the last telescope objective made by Fraunhofer was supplied for a transit telescope at the City Observatory, Edinburgh,, Astronomical Society of Edinburgh the telescope itself being completed by Repsold of Hamburg after Fraunhofer’s death.

Invention and scientific research

One of the most difficult operations of practical optics was to polish the spherical surfaces of large object glasses accurately. Fraunhofer invented a machine which rendered the surface more accurately than traditional grinding. He also invented other grinding and polishing machines, and introduced many improvements into the manufacture of the different kinds of glass used for optical instruments, and which he found to be always injured by flaws and irregularities of various sorts.

In 1811 he constructed a new kind of furnace, and on the second occasion when he melted a large quantity found that he could produce flint glass, which, taken from the bottom of a vessel containing two hundredweight of glass, had the same refractive power as glass taken from the surface. He found that the English crown glass and the German table glass both contained defects occasioning irregular refraction. In the thicker and larger glasses, there would be more of such defects, so that in larger telescopes this kind of glass would not be fit for object glasses. Fraunhofer therefore made his own crown glass.

The cause which had hitherto prevented the accurate determination of the power of a given medium to refract the rays of light and separate the different colors which they contain was chiefly the circumstance that the colors of the spectrum have no precise limits, and that the transition from one to another is gradual and not immediate; hence, the angle of refraction could not be accurately measured. To obviate this, Fraunhofer made a series of experiments for the purpose of producing homogeneous light artificially, and unable to effect his object in a direct way, he did so by means of lamps and prisms.

Fraunhofer demonstrating the spectroscope.

Thus in 1814, Fraunhofer invented the spectroscope. In the course of his experiments he discovered that bright fixed line which appears in the orange color of the spectrum when it is produced by the light of fire. This line enabled him afterward to determine the absolute power of refraction in different substances. Experiments to ascertain whether the solar spectrum contained the same bright line in the orange as that produced by the light of fire led him to the discovery of the 574 dark fixed lines in the solar spectrum.See:

  • Joseph Fraunhofer (1814 – 1815) (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), Denkschriften der Königlichen Akademie der Wissenschaften zu München (Memoirs of the Royal Academy of Sciences in Munich), 5: 193-226; see especially pages 202-205 and the plate following page 226.
  • Reprinted, with additional findings and notes, in: Joseph Fraunhofer (1817) (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), Annalen der Physik, 56: 264-313; see especially pages 278-286.