How to see the spectral lines of the Sun

Joachim Köppen Kiel/Strasbourg/Illkirch Spring 2007

  1. Choose a room which you can darken by drawing the curtains or closing doors, except for a small section of a window which receives light from the bright sky or off a white sun-lit wall. It is not necessary to have direct sunlight shining in.
  2. Cut with a sharp knife in a sheet of paper or cardboard a slit (about 1 mm by 5 cm) with clean edges. Place it on the window, with the long side of the slit oriented vertically.
  3. Stand near to the slit with your back towards it so that the light passes over your shoulder.
  4. I did my first experiments in my appartment's corridor, where I closed all doors, and I opened slightly the door to the kitchen so that the gap was illuminated by the light from the kitchen window. Thus I had plenty of space in the corridor to move around and look at the CD from various angles ...
  5. Hold a CD in front of you and orient it so that you can see in it the white reflected image of the slit. You may use any CD - whether it contains music or software does not matter - but for best results I recommmend a recordable CD ROM (fresh or with data - it does not matter) which is apparent by its more brillant reflections. A silvery colour is preferred, but the bluish or greenish ones also work.
  6. Orient the CD so that the slit appears in a part where the grooves are parallel to the slit. Check whether the whole slit is brightly and evenly lit. If necessary, change your position or orientation.
  7. Now swivel the CD about a vertical axis either to the left or the right until a bright rainbow-coloured band appears. The violet colour comes first. The band which is on the side of the disk's hole has a smaller vertical extend than the one on the other side, but it tends to be clearer, and I suggest that you try this first. If you follow the sketch, you should turn the CD towards the right.
  8. In the coloured band you may be able to see several fine dark vertical lines. The strongest is one in the orange: the Sodium "D" line, but the green Magnesium "b" line is also easy to see. This is a bit tricky, and you want to change the distance between your eye and the CD to get a clearer view. The best results are obtained when the total distance from the eye to CD and from CD to the slit is a comfortable viewing distance for you.
  9. These lines were discovered in 1814 by Joseph Fraunhofer. In 1859 Gustav Kirchhoff and Robert Bunsen found that each chemical element has characteristic spectral lines. The lines in the Sun and stars are formed by absorption in the upper parts of their atmospheres. They tell us about the temperatures, surface gravity, and chemical composition of stars.

All this procedure with a dark room is a bit tricky and tedious. It is much more comfortable to build oneself a spectroscope and look at the spectrum of the sun or any other light source whenever and wherever one wants: see here for instructions and designs for various spectroscopes.

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