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Image: Pipevine Swallowtail has pretty orange spots

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Description: Like many of the black swallowtails, the Pipevine Swallowtail has pretty orange spots. The orange color is caused by deposits of pigment. The Pipevine also glows an iridescent blue color when viewed in bright sunlight. This is not caused by pigment but rather by a process called Thin-Film Interference (TFI). The process of TFI is dependent on the angle of the light hitting the butterfly. I have found through trial and error that early morning and late afternoon are the best times to catch this blue color. I assume it is because the sun is lower in the sky at these times and hits the wings near to the perpendicular. Mid day it is harder to catch the blue color with as much intensity. The above shot was taken close to 10:00 am which is about when the butterflies really begin to appear and, coincidentally, is about when I have had enough coffee to venture outdoors. Thin-film interference (TFI) is a process that causes certain frequencies of reflected light to cancel (destructive interference). This occurs when light reflects from inner and outer layers of a surface. When the distance between the membrane surfaces is close to the wavelength of visible light, the process of TFI can turn white light into the colors of the rainbow. TFI is the same phenomenon that causes color in soap bubbles and oil films. To view the phenomenon in the Pipevine it is handy for them to land with wings facing the sun. Below I demonstrate and discuss the phenomenon of "thin-film interference" or TFI. Even with a degree in Physics I have a hard time understanding the math but the phenomenon is easy to demonstrate in the Pipevine Swallowtail. It is far better to see it in real life.
Title: Pipevine Swallowtail has pretty orange spots
Credit: Thin-Film Interference
Author: John Flannery from Richmond County, North Carolina, USA
Usage Terms: Creative Commons Attribution-Share Alike 2.0
License: CC BY-SA 2.0
License Link: https://creativecommons.org/licenses/by-sa/2.0
Attribution Required?: Yes

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