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Fly Wing Interferencies

On May 14, 2012, Daniel Gerstgrasser photographed beautiful interference colours on the wings of a fly near Zürich. (Camera Canon , 100-mm-macro-lens, f11/125sec, ISO 100). The colours are caused by interferences. The structure of the wings which consist of thin, stacked layers with different refraction indices reflect the light from each boundary where the refraction index changes. So the incoming light is split up into a wave reflected from the front side of the layer and another wave reflected from its rear side. Both waves meet in front of the layer causing the interference.

This is because one wave has to pass through the layer while the other one is already reflected from the front side. So both reflected waves are shifted against each other due to the difference in time they need for these different ways. As the different colours are caused by a difference in light wavelength, it depends from the colour of the light if this shifting causes an amplification or an extinction of the respective colour. If, for example, red is extinguished, the complementary colour, which in this case is green, remains. So the wing shimmer greenish.

Light crossing the layer in a flat angle has to cover a longer distance inside the layer. This causes a different shifting against the wave reflected from the front side of the layer than it is compared to light leaving the layer in a more pointed angle. This is why there appear different colours when looking at the wing from different directions.


Bird Feather Iridescence

While taking a walk through the surroundings of my home on February 20, 2010, I took the most of the nice weather by taking some last winter photographs. At 10:09:27 CET, a small covey of about 15 siskins (Carduelis spinus) flew off an alder in front of me and passed me to the right. Seen from my position, they directly passed in front of the sun. I took some photographs with my Sony DSLR-A 700 and a Minolta lens 4/300 mm. The exposure time was 1/8000 second at an aperture of 32 and ISO 200.

Further settings of the camera were: Programme, serial photographs, automatic white balancing, and integral measurement stressed on the centre of the picture. In the original photograph, the sun is almost at the centre of the photograph. The precedent image of the series was exposed for about 1/4000 second at an aperture of 16.

That picture is brighter (a small part of the sun can be seen at the right rim of the photograph!) and the iridescence in the feathers looks rather faint.

Author: Rene Winter, Eschenbergen, Germany

Animal Optics

In this blog we have already seen photos of spiderwebs scattering rays of light, and even a thistle seed showing spectral colours. Now I would like to contribute to this collection with another possible source for observing this optical phenomenon: cat’s hair. The pictures attached show the dark Sámán (of Mónika Landy-Gyebnár) and the white Manci (my cat). When light comes from the appropriate angle, their fur can display the colours of the rainbow surprisingly clearly. Please, click the images to see it better. Full size photos of Sámán’s and Manci’s shining hair are also available.

The other photo is another “animal test”. Given that birds’ feathers have a very regular structure, I was interested whether they show corona-like diffraction if we place one in front of a light source. I found that the repetitive structure of barbs, barbules and tiny hooks interlocking on them create beautiful interference patterns. This picture – somewhat out of focus – shows an owl’s feather (a secondary from its wing) with a lamp behind it. The pattern is curved because the feather itself has a bent surface. Further images: #1, #2, #3

Author: Ágnes Kiricsi, Hungaria