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Name: Marty G.
Status: Educator
Age: 40s
Location: N/A
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Date: N/A 

What causes the "slicks" that sometimes form on a large, irregular body of water with flow (river or estuary, for example)? The slick appears under certain wind conditions as a linear, with-the-current calm region on an otherwise more ripply surface. (I'm guessing that a very thin layer of oil or oil-like material is stretched along the surface, decreasing the wind's purchase on the water.)

Your guess is probably correct. The "slicks" are visible because of light interference because the film thickness is of the order of the wavelength of visible light -- 400-700 nanometers. The source could be from agricultural runoff, oil contamination or even a "natural" source such as the sap from evergreens. The colors of the slick will move and swirl with the current as the moving water changes the thickness of the film.

Vince Calder

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