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Re: Rainbow interfernce ?



I finally got a chance to see the disputed photograph and I'm with
Ludwik and Robert Cohen.

The dark region between the double rainbow is simply a region that is
impoverished in scattered light. It is not an interference effect.
The primary rainbow is a caustic formed by light that internally
reflects once and ends up being scattered at all observation angles
from directly opposite the sun "up" to the arc. The secondary
(inverted) rainbow is a caustic formed by light that internally
reflects twice and ends up being scattered at all observation angles
from directly toward the sun "down" to the secondary arc which is
located above the primary arc. The region in between the two arcs is
left *relatively* dark.

There *is*, however, a very interesting interference effect in
rainbows that produces what is called "supernumerary arcs." Briefly,
it is caused by interference between the two wavelengths that are
scattered in any given direction from a single raindrop into
directions in the lower region of the primary rainbow. It produces a
complex series of fine bands in a variety of nonspectral colors at
the lower edge of the rainbow. Because the effect depends on
raindrop size, it tends to be washed out unless the raindrops are
relatively uniform in size.

We've touched on this subject here before. See my observation and
question in July 1996

<http://lists.nau.edu/cgi-bin/wa?A2=ind9607&L=phys-l&D=0&P=2638>

and Alex Burr's response

<http://lists.nau.edu/cgi-bin/wa?A2=ind9607&L=phys-l&D=0&P=2700>

Google is, of course, the best portal to the subject.

<http://www.google.com/search?q=supernumerary+rainbow+interference>

--
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm