The Spectrum

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Why?

The wider the angular dispersion, the more spread out the colors are, and the more detail is apparent

Prism improvers used various kinds of glass, even liquids, and different geometries to improve the resolution

Slide 13

Joseph von Fraunhofer

German Optician b. 1787

At age 11, he was apprenticed to a glassmaker, Philipp Anton Weichelsberger

Weichelsberger’s shop collapsed, but both were rescued

Maximillian IV, Prince of Bavaria witnessed the rescue and took him under his wing, providing him with books on physics and mathematics

Maximillian presented him with a sizable contribution to buy his way out of apprenticeship at age 19, so he went to work for lawyer named Joseph von Utzscneider who had entrepreneurial aspirations

In partnership with Utzscneider he strived to make superior optical devices, including better lenses for telescopes and wide dispersion prisms

In 1814, searching for a pure light with which to calibrate his instruments, Fraunhofer turned his prism to the Sun

Slide 14

What are these lines?

Slide 15

Fraunhofer expected a pure, continuous spectrum but was *surprised by the dark lines

Saw similar spectra in the light of Sirius

1821: Developed a superior diffraction grating

Invented the spectroscope

The basic design is still used

1822: Became keeper of the museum for the Royal Academy of Sciences in Munich

Died 1826 of tuberculosis, age 39, before Bunsen and Kirchhoff offered an explanation for the lines later in the century

*William Hyde Wollaston saw the same lines in 1802 but wasn’t impressed

Slide 16

Even though Fraunhofer used his spectroscope on a star*, there was no such thing as stellar spectroscopy at the time

Astronomy was all about position, motion, cataloging, discovery of new objects

Little or no significance was given to the composition of celestial objects

*The Sun was not considered a star until later in the 19th century

Slide 17

[Astronomy] must lay down the rules for determining the motions of the heavenly bodies as they appear to us from the earth…Everything else that can be learned about the heavenly bodies … is not properly of astronomical interest.

Friedrich Wilhelm Bessel

(1784-1846)

Slide 18

In astronomy, human ingenuity will, probably, in future, be able to accomplish little more than an improvement in the means of making observations, or in the analysis by which the rules of computation are investigated. —John Narrien (1833)