Pine Tree Oil on Lake Water
John Stetson captured the scene. Overhanging pine trees have dropped their oil onto the lake producing thin fragmented films.

interference colours.

Light reflects from both the top and bottom of the oil film. The two reflected waves combine and the appearance of a colour depends on the extent to which the two waves of that wavelength are in phase. The phase, determined by the two waves' path length difference alters with film thickness, colour and viewing angle.

The brightest interference colours are from thin films, ones a few wavelengths thick. In thicker films the path length difference changes rapidly with angle and the colours get washed out.

Image ©John Stetson, shown with permission
Outgoing waves happen to be in phase in this sketch. Alter the wavelength, angle or film thickness and this changes.
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oil film

lake water