Saturday, September 30, 2006

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.

Wednesday, September 27, 2006

New results on the large coronas on water surface

Some time ago Jari Piikki posted me a couple of drops of the surface film that produced his colourful corona. I mixed it in a bucket full of tap water with also some forest litter added. After a week or so there appeared a bluish film on water surface. Closer inspection with point-like light revealed more colors. It was actually a large corona, with blue inside, and green and red on the outer edge.

Only a segment of the corona could be seen in the small bucket. This is shown in the upper left image. Unlike the peculiar corona described by Piikki, this one has true blue aureole starting right from the reflection image of the lamp. Outside the blue some green and perhaps red maybe distinguished – visually these colors were clear. Upper right image is microphotograph of the film. It consisted of bacteria.

Day after taking the upper photographs the phenomenon in the bucket changed. Now there was similar corona as what Piikki photographed, only less bright. This is shown in lower left. There was no aureole, but instead a dark area surrounded the light source. This also was consistent with Piikki’s observation. Microphotograph of the film on this stage is shown on the lower right.

The most conspicuous difference in the bacterial level between the two stages is the crowding – there are more bacteria in the latter stage. Is this somehow related to the changes in the phenomena? Perhaps Minnaert can give an answer. In the chapter about coronas seen in windows, he describes how these coronas have an aureole replaced by dark area. He speculates the dark area forms because in the two dimensional plane of the window surface the droplets can be evenly spaced – a situation which can not happen when particles are freely floating in the air.

Now, if one looks at the lower right microphotograph, one can imagine that these bacteria indeed are spaced more evenly as compared to the upper photo. In the Piikki’s microphotograph, which looks like a maze, the bacterial film might consist solely on the long strands that are seen in small numbers also in my lower right photo. When packed next to each other, these kind of strands would possibly be very evenly spaced, thus perhaps contributing to the stronger corona. Piikki’s photograph is, however, too much out of focus to be sure of this interpretation.

The bacteria species could be Nevskia ramosa, which is well known neustonic (water surface inhabiting) bacteria. Genomic fingerprinting would be needed for confirmation. For the last note it should be said that the inverted colors between the two microphotographs are due to differences in focus. The upper image is a bit out of focus and thus the bacteria shows as dark spots.

Tuesday, September 26, 2006

Polarized fogbow in car headlights

I spent the past summer at Langmuir Laboratory on the Magdalena Mountains, in southwest-central New Mexico (USA) at an elevation of 3.2 km. The purpose of this was thunderstorm research. The monsoon here was unusually wet and on several days and nights the mountain laboratory was actually foggy. This is relatively rare considering the New Mexico climate. I took this opportunity to view polarized fogbows in my car's headlights, and on September 2nd, I was particularly successful.

When I programmed a Mie simulation algorithm late last year and plotted a polarized fogbow on my screen, I was surprised that the polarized bow looked as it did, with the typical Brewster's angle 'gap' in the main bow for parallel polarization. How excited I was to see that the actual fogbow indeed looked like the simulation! I had never seen it before in nature.

I am sure this has been done before by someone else, but I thought I would post the images anyway.

I covered up one of the car's headlamps as to not have a double bow. I positioned myself about 50 meters in front of the truck, which I had parked on a slight inclination so the bow would be better visible against a featureless sky and be more complete. The fisheye lens was equipped with a polarizer at the place in the lens where the rays go parallel.

The simulation I made earlier, for a 10 micrometer radius droplet. It looks sharper because I assumed a point light source, assumed a monodisperse droplet distribution, and it was not divergent light. It is not a perfect match either considering the placement of the supernumeraries: probably the droplets in the actual display were a bit smaller. Because of the divergent light source, and because I don't know the distance to the truck accurately, I doubt I will ever be able to accurately tell the actual droplet radii in the display.

The polarized glory was also obvious, but my shadow was blocking most of the part that was most polarized. I am including the unpolarized glory here.

The close-ups of the polarized and unpolarized fogbow were made with a 24mm/2.8 lens. The camera was a Canon 300d (modified version - i.e. with IR filter removed). I did not need to adjust the brightness and contrast much to get the results as displayed here. The fogbow had good contrast by itself.

About 10 days later I documented a natural fogbow in sunlight from the laboratory, through a polarizer. I photographed that with film; I have not processed those photos yet.

Tuesday, September 19, 2006

Crepuscular ray 1 hour after sunset

On September 12th Claudia Hinz observed the sunset from the 1835m high Wendelstein Observatory.

At 19.18 CET (Daylight saving) the sun disappeared behind a 250 km distant thunder cloud over the Black Forest, Schwarzwald, in South Eastern Germany. Then the sun would have been ~2° above the astronomical horizon. The sea level horizon was 197 km distant and dipped 1.4°.

At around 20.00 sheet lightning was noticed on the horizon.

At 20.15 a 30° long crepuscular ray was formed by the thundercloud. At that time, nearly an hour after sunset, the sun was 8° below the astronomical horizon. The ray was faint but clearly seen over the remaining twilight colours. The image was made with a 150mm lens and a 4s exposure.

Claudia Hinz

Wednesday, September 13, 2006

Divergent double rainbow

On the night of Dec 12 2004 the weather was was quite murky and the rain was unusual because it was a misy where the drops were quite small but large enough to be felt. That night I turned on my car's headlights and got this stunning set of divergent light rainbows. In the first photo you can see a single set of bows and the second shows three possibly 4 bows due to the camera being positioned between the two headlights.

Monday, September 11, 2006

Water colours at Juva & a new optical phenomenon

Jari Piikki have also found some ponds with algal films that show optical phenomena. The ponds were sampled and the alga allowed to breed for some weeks. Extracts were photographed through a microscope. In one pond, Botryococcus alga was abundant nine years ago but then disappeared. Now it is back and quite large Quetelet rings and a corona were visible. Another pond on one island possibly contained Chromulina rosanoffi and microscope images showed it floating on the water surface.

Jari Piikki also found a new optical phenomenon. He took (like Marko) some samples from ponds and cultivated them. In one of them Chromulina disappeared and some other alga appeared. There could be seen a very colourful ring with two zones of spectral colours around the reflection from the surface of the Sun or an artificial light. The ring was oval and became smaller, when the light was brought closer to the water surface. Its diameter in sunlight was about 30 degrees and the inside was dark, so it was not an aureole. also visible when the light shone through the alga from below. Many photographs of the surface were taken with a microscope to show its structure. You can see it in some photos. The alga constantly changes constantly and now shows small colourful blotches.

[Text: Jari Piikki]

Saturday, September 09, 2006

Optical phenomena from algal film on water surface

Here are some results from the work I have done this summer on the phenomena caused by algal film on water surface.

The upper pair of photographs shows elliptical corona and the alga that caused it. Pine pollen gives the scale. This corona-type display has no other phenomena.

The lower photo shows another type of display, which exhibits Quetelet rings, brightenings around sun's reflection image, glory and fogbow. In this case there is also unidentified diffuse feature marked with arrows. In addition, fogbow and glory have spot-like brightening towards nadir. Two B&W photos of the algae causing this type of display are shown. In the lower photo the algae are possibly turned 90° in comparison to upper, showing elliptical shape. This species might be Chromulina rosanoffii. It's about 5-10 ┬Ám in size (not to scale with upper color photo) and well transparent to make glory.

These phenomena turned out to be common, at least this summer. They were most abundant on rocky Finnish archipelago islands, which harbour endless number of small rock pools.

Friday, September 08, 2006

divergent light rainbows

Fogbows have a similar origin to rainbows. For this reason, Christian Fenn, who had previously photographed fogbows made by divergent light, decided to attempt to image a divergent light rainbow. On 19th April in Hammelburg, Bavaria he managed, in pouring rain, to image a rainbow formed by light from car headlamps.

A divergent light source can actually produce a multiplicity of rainbows, not only of angle 42° but at larger angles also. The net result is that the bows overlap and a discrete coloured arc is no longer visible. Another negative factor is that the rainbow cannot develop a high intensity like those sourced by the sun because only a narrow range of rays fall on the "rainbow cone" having its tip at the observer's eye. To see a divergent light bow it is necessary to be far away from the light source so that its rays are as parallel as possible and develop a bow of sufficient contrast.

In the photograph the divergent light bow is wider horizontally than vertically. This is because the two car headlamps each form bows and so produce an apparent broadening.