Pure Spectral Colours
A circumhorizon arc lights high cirrus cloud over the Czech Republic 6th June '14. Images by Roman Szpuk.
All images ©Roman Szpuk
Spectroscope in the Sky

A simple prism spectroscope has light made parallel by a slit and lens entering a glass prism. Rays of differing wavelength or frequency (which our brain perceives as colours) are then deviated (dispersed) at different angles. The result is a spectrum, red to blue and violet.

Circumhorizon arcs (and their close relatives circumzenithal arcs) are Nature's spectroscopes. In the CHA near parallel sunlight enters a flat plate ice crystal. When well behaved the crystal is aligned horizontal to within a fraction of degree - almost as good as the clamp holding the glass prism. The rays pass in effect through a 90° ice prism where the component colours are deviated as in a spectroscope to form a rather pure spectrum.

A reasonable spectroscope will show dark lines crossing the sun's spectrum**. These Fraunhofer lines are absorptions at specific frequencies by sodium, calcium, hydrogen and other elements in the sun's photosphere. Could we see them in a CHA or CZA? Doubtful because the light from the sun's 0.5° diameter disk is not parallel enough. But perhaps worth the try?

**With care an old CD and polished knitting needle
   will show them.

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