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Reflected Sunlight Dewbow


Reflected Sunlight Dewbow. Photos: Jérémie Gaillard

On August 19, 2010, Jérémie Gaillard made an interesting discovery when looking at the surface of the lake Etang de l´Alleu which is located in the French community of Saint-Arnoult-en-Yvelines. The water was covered with pollen, on which droplets of dew had formed. In these droplets two colourful rainbows were visible. Dewbows can be understood as the lower part of a rainbow projected onto a horizontal plane. When a dewbow is fully developed, a semi-circle which opens towards the sides should be visible, the apex of which is situated at the lower end of the observer´s shadow. Equivalent to normal rainbows, primary and secondary dewbow should run parallely, but in Jérémie Gaillard´s observation they did not.

Instead, the second colourful bow fragment is a reflected sunlight dewbow. The surface of the water acts as a large mirror reflecting the sun. The reflected image of the sun now acts as a second source of light, which is situated as far below the horizon as the sun is above it. (angle of incidence = emergent angle). So the antisolar point for the reflection of the sun is above the horizon. This reflected antisolar point, which is located the double of the real sun´s elevation above the antisolar point, is the centre of the two rainbow circles for the reflected sunlight. So the additional rainbows are displaced upwards by the double sun elevation compared to the primary and secondary rainbow, making a rather unfamiliar appearance in the open nature.

Author: Claudia Hinz

Algae optics in Wisconsin

While on my 4-wheeler riding around in the wood in Lakewood Wisconsin I came upon a puddle with a green film in it in the middle of a dirt road and I drove up to check it out and I am glad I did! As I was dismounting from my 4-wheeler colors on the puddle caught my attention and I found the same thing Marko Riikonen found algal optics. Over the period of 10 minutes or more I observed corona similar to pollen corona around sun’s reflection and Quetelet rings which I have always wanted to see. I also saw glory with the similar brightening’s like pollen corona and a white fogbow like glow around the glory. More pictures here.

Author: Michael Ellestad, Wisconsin, USA

Reflected light dewbow on water surface

I and my wife Eliisa Piikki got a hint about a light phenomenon on a lake nearby. Eliisa took some photos and there was a dewbow and a reflected light dewbow in those photos. They were formed on the waterdroplets that were settled down on the Chrysomyxa ledi needle rust (Small-spored spruce-Labrador-tea rust).

That rust can’t be seen in Finland every year, but this year it is very common especially in the Eastern and in the Northern Finland. Because of the rust the young needles of some spruces are brown.

Author: Jari Piiki, Finland

Thin film interference on the water surface

In the city of Lahti there is a landfill hill from which base in some places flows iron rich water. The bottom of these ditches is rusty brown and water has a strong iron smell. In some locations the water surface is completely covered with a film that displays spectacular colors in cloudy weather. This film is caused by iron oxidizing bacteria. The bacteria itself resides in the water, but it produces on the water surface substances from which it hangs down like a chain of sausages. Iron oxide is one of these products and it may be the cause of colors.

Author: Marko Riikonen

Reflexion on water surface

Brigitte Rauch and her husband spend every free minute on their motor-boat on the waters around their home island Helgoland, whichis only about one square km large and situated in the open North Sea. If possible, they do not miss any sunset and always have their camera ready to hand. This photograph of reflections on the water surface was taken on July 17, 2006, just after sunset. The colour of the sky and the clouds are reflected from the water surface, which was relatively smooth that evening. The patterns are caused by small waves. This richly coloured interplay of sky, clouds and waves can be seen only on a few days of the year.

More pictures: 1 2 3 4

Small coronas from bacterial film on water surface

Most of the cultivation pools in my room are now showing much smaller coronas than earlier. In each pool the corona is of constant size over the whole bacterial film surface, but from pool to pool their size vary.

Even though these coronas are smaller than before, they still are large as compared to pollen coronas. Unfortunately I have no photos for comparison.

In the composite image are two coronas photographed with same lens (not to scale with upper single image, which has been taken with zoom lens). The microphotograph of the bacterial film is from the smaller corona on the left.

The light source was a 50 W halogen spotlight that I made even more concentrated by placing the lamp in a cardboard box, into which a small hole was made for light to come through. When taking photos, the room was otherwise darkened.

[Posted by Marko Riikonen]

Chromulina rosanoffii as a cause of optical phenomena on water surface

Excavating the literature finally gave confirmation that it is the alga Chromulina rosanoffii that causes Quetelet rings and glory on water surface. This unique sort of alga separates itself from the water surface by forming a stalk on top of which it rests. This is seen in the right hand microphoto which is taken parallel to the water surface. Light blue is air and black is water. On the left is a photo taken at right angle to the water surface, thus giving a look at the C. rosanoffii forest from above (transmitted light). If the alga is for some reason submerged in the water, it sheds its stalk and starts immediately swimming. The b&w photos here show C. rosanoffii submerged in water.

Also two photos of the optical phenomena caused by this alga are displayed in the composite. On the left is a fisheye view with Quetelet rings. The glow around the camera shadow is fogbow. Notice the brightening towards nadir in fogbow, indicating possibly non-spherical particles. The photo on right is taken further away from the cultivation tub, showing spectral colored glory rings. Attached to the outmost glory ring is faint, white fogbow.

All said here about the biology of C. rosanoffii was known long time ago. The alga was described in 1880 by Russian botanist Woronin, who encountered it while visiting Finland. One aspect of the studies carried since has been the golden glow that well developed C. rosanoffii surface film displays, as shown beside and here. But I have not yet met in the literature any mention of the spectral colors, which strikes me as a small wonder. However, there is still plenty of reading to be done, so something may come up.

[Posted by Marko Riikonen]

Colourful refraction by the surface waves

On June 13th Thorsten Falke from the German North Sea isle “Düne” (near of isle “Helgoland”) was walking along the seashore, when he saw criss-cross light structures on the seabed obviously caused by refraction by the surface waves. He had seen the phenomena often before but this time it seemed to be coloured. He decided to paddle out into the water to take pictures. It was difficult to see the colours visually because of the rapid movement of the little waves (only 2 cm “high”) but in many of the pictures the colours were evident. The appended picture was taken with a macro lens and shows a shell in 10 cm deep water with a colourful refraction band across it. Because of the macro lens the picture is nearly 2 times life size. Another refraction is located at the upper edge of the picture. If he remembers rightly, the light was coming from the left side and was parallel to the wave crests. The main interest of Thorsten is to observe whether the colourful refraction is seen only when the wave crests are parallel to the light direction or if it is also visible when the crests are at right angles to the sunlight.