JOSEF VON FRAUNHOFER (1787-1826)

1823 – Germany

‘The spectroscope’

A significant improvement on the apparatus used by Newton. Sunlight, instead of passing through a pinhole before striking a prism, is passed through a long thin slit in a metal plate. This creates a long ribbon-like spectrum, which may be scanned from end to end with a microscope.

image of the visible portion of the electromagnetic spectrum showing a series of dark fraunhofer lines

Cutting across the ribbon of rainbow colours are thin black lines. The lines are present even when a diffraction grating is used instead of a prism, proving that the lines are not produced by the material of a prism, but are inherent in sunlight.

An equivalent way of describing colours is as light waves of different sizes.
The wavelength of light is fantastically small, on average about a thousandth of a millimeter, with the wavelength of red light being about twice as long as that of blue light.

Fraunhofer’s black lines correspond to missing wavelengths of light.

By 1823 Fraunhofer had measured the positions of 574 spectral lines, labeling the most prominent ones with the letters of the alphabet. The lines labeled with the letters ‘H’ and ‘K’ correspond to light at a wavelength of 0.3968 thousandths of a millimeter and 0.3933 thousandths of a millimeter, respectively. The lines are present in the spectrum of light from stars, usually in different combinations.

Fraunhofer died early at the age of 39 and it was left to the German GUSTAV KIRCHHOFF to make the breakthrough that explained their significance.

Astronomers today know the wavelengths of more than 25,000 ‘Fraunhofer lines’.

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