Re: Consider a soap bubble.

I probably have an advantage because I already knew the answer, but I also think 
it is possible that you could have found the answer if you used the Web. Doing a 
search on the terms: 
 +"soap bubble" 
 +interference
 +color 
on AltaVista I found this link  to
the Exploratorium, one of the most fascinating places in the World. It answers 
the question of where the colors come from.

Here is an answer from another excellent site:
How do oil spills/spots (i.e. in parking lots and streets) create rainbows?

A thin layer of oil on water creates interference effects, just like those seen 
in a thin soap film. Sunlight reflects from both the top and the bottom of the 
oil layer and these two reflections can interfere with one another. If the blue/
green wavelengths of light interfere destructively on their way to your eye, you 
will see the oil layer as red. If the green/red wavelengths of light interfere 
destructively, you will see the oil layer as blue. How you see the oil layer 
depends on its thickness and the angles of the light.
***end of quoted material

You may want to see if your bookstore or your library has this reference: The 
Science of Soap Films and Soap Bubbles. I got mine at Borders for $9.95. A real 
bargain because it has so much great stuff on optics and physics of soap 
bubbles. The black area that you mention occurs when the thickness of the bubble 
is less than 300 angstroms. This is less than the wavelength of visible light so 
no interference effects can create the colors that thicker films allow.

David Winsemius, MD