Discussion of the Question 08/99
ZEROTH ORDER RAINBOWThe question was:
A ray of light coming from the sun (A) enters a water drop, is reflected internally once, and exits in direction B. The angle of maximal intensity of this reflected light depends on the index of refraction of the water and thus is slightly different for different colors. This gives rise to the primary rainbow at about 42 degrees opposite to the direction of sun. (The secondary rainbow corresponds to two internal reflections.) Question: Why can't we see a zeroth order rainbow by looking at rays like C that exit the drop without internal reflections?
(8/99) Dr. Fred Goesmann from Max-Planck-Institut fuer Aeronomie (Katlenburg-Lindau) (e-mail goesmann@linmpi.mpg.de) wrote:
Are you sure such a rainbow is never seen? A few weeks back I saw something which would fit. It was near sunset and coloured bands were visible to the right and to the left of the sun (I do not recall the angle, but I would guess it to be about 20 degrees to either side, maybe less). The coloured patterns were only visible near the horizon and on the same height as the sun. Of course one would probably need a highly transparent thin curtain of droplets through which the sun is seen, but is this outright impossible? I do not think so.
Y. Kantor: I am not sure what is described in the above observation. However, I am guessing that these were coronae - concentric colored rings repeating around the sun or moon several times(!) with colors going from white to blue to green, yellow and red and repeating. This is caused by interference of light diffracted in drops of water. This effect should be distinguished from (much stronger) rainbows caused by geometrical optics effects. Beautiful pictures of coronae can be found in the book Color and Light in Nature, by D.K. Lynch and W. Livingston (Cambridge Univ. Press, 1995).
We welcome comments on the subject.
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