Colours in Citric Acid

Lately I experiment a lot with crystal growing and I knew that Citric Acid can refract the light, but when I saw this I was more than surprised. These flat crystals of about 1-2 cm in diameter (the larger ones) were grown between two glass plates, then put in front of a polarized light source and photographed with a polarization filter. It took a few tries to find a proper way to grow them flat enough for this purpose though. More Photos see here and here.

Author: Rolf Kohl, Germany

Spiderweb-Halos

Trying to photograph a spiderweb against the sun to get a completely forward-scattered image of it led me to the discovery of the “spiderweb-halos” (Foto 1). I got a rather dim image of the web against the darker background, but I was struck by a much brighter circle-like structure. This was the moment of the discovery of “spiderweb-halos”.

All you need to get them is a spiderweb between yourself and the sun and a device to block the sun. I used my self-made filter usually used when photographing halos, made of a small piece of black slide film inserted on one end of a ball pen’s tube. It has the right transparency to show also the contour of the sun disc on the photographs (Foto 2). One of the best pictures I get is this (Foto 3).

The spiderweb-halos belong to the general phenomenon called “Circles of light in treetops” (M. Minnaert, Light and Colors in the Outdoors (1993), §29: Foto). As seen on the pictures, the bright forward scattering part of each thread is that one, which is perpendicular to the sun. These segments form an (interrupted) circle between the spiderweb centre and the sun.

Taking several pictures I discovered the dynamics of the spiderweb-halos (Foto 4). The diameter of the circle increases with increasing distance of the sun from the centre of the web (Foto 5), until the halo becomes a dim and open circle segment when the sun is far outside the web (Foto 6). With the sun in the centre of the web, the whole web will be lit up as a “full halo” (not perfectly realized in Foto 2).

An outer circle segment may also be seen, depending on the construction of the web (Foto 4). Another feature of the spiderweb-halos known to halo observers is the “pillar” between the sun and the center of the web. The pillar may vary in appearance from a truly pillar-like structure (Foto 7) to a triangular segment (Foto 8). The pillar may also be seen outside of the sun (if the sun is located near the centre of the web; Foto 9) and extend to the other side of the center of the web (“counter-pillar”, Foto 7).

The full-scale of  “spiderweb-halos”  is shown in this very schematic diagram – it visualizes also why I termed them “halo”: all appearances are known also to the atmospheric halo observer.

Author: Christoph Gerber, Heidelberg, Germany

Spider silk glitter path

While taking a walk, I noticed a field that was covered with fine spider silks. The sun was rather low (about 10°) and made a kind of lower light pillar appear in the silks. When I took a closer look, I could also see concentric circles of light with the sun in their centre.(2)

These circles are caused by the perspective and the angle in which the light strikes the surface of the spider silks. The light gets reflected best to the observer when the reflecting surface is positioned at an angle of 90° to the source of light. A similar effect can be observed when a street lamp shines throug wet branches of a tree.(3)(4)

The “lower light pillar” can be seen better because the sun as the source of light shines vertically down onto the field and all spider silks in this direction reflect the light towards the observer.

Place : Kämpfelbach, Germany
Time : 01 November 2011
DSLR Camera : Canon EOS 450d
Exposure : 1/60 sec, f/22mm, F/10, ISO 100

Author: Michael Großmann, Kämpfelbach, Germany

Noctilucent clouds in october ?

Foto: Rüdiger Manig, Neuhaus, Germany

In the evening of October, 21, 2010 between 18.45 and 19.15 CEST in Czech Republic, Austria and Germany spreads of clouds were observed from the northwestern to southwestern horizon at a maximal altitude of 15-20°, which reminded very strongly to noctilucent clouds (NLC). But NLC is not ineligible as explanation for this season of course and also the cloud structure’s altitude calculated from the sun elevation (-8° to -11°) is ca. 60-70 km is too low for NLC.

All known observations with observed direction of clouds (if noted).
Black Points: Observations without photos
Red points: Observations with photos (meteoros-forum)
Violet points: Animations (1 – 2 – 3)

The most probable cause could be the exhaust gases of the Soyuz rocket, which was launched in Kourou, French Guayana at 12.30 o’clock, in order to bring the first satellites for the European navigation system Galileo into the earth orbit. During the overflight above Europe a large extent of water, carbon dioxide and nitrogen containing exhaust gases were ejected with a speed of approximately 4km/s and fell downwards unimpeded. At a height of approx. 70km the water vapour condensed into clouds, which were illuminated -  like the NLC – also by the sun, while normal clouds lay long in the shade.

A further possibility of explanation could be a meteor trail. There is one not too much detailed observation, which could refer to a fireball: Report in German and with automatically Bablfish-Translator. But this possible fireball was in this moment, in which would observe the NLC-shaped clouds (18:45 CEST). And as experience teaches a meteor trail needs several hours to develop.

Authors: Landy-Gyebnár Mónika, Claudia Hinz, Wolfgang Hamburg

Circular Wing Tip Trails

During a helicopter operation at Mt. Wendelstein on September 9, 2011 in very moist air, Harald Jauernig observed that the rotor blades of the helicopter caused condensation of the moisture.

This phenomenon is called wing tip trails and is caused by the difference of pressure between the lower and the upper part of plane wings. The air passing above the wing moves faster and so has less pressure than the air passing below the wing, causing an updraft which lifts the plane. This is what makes airplanes fly.

When pressure drops, the air cools down, so that moisture can condensate and form droplets or even ice crystals when it is cold enough. These become visible as a thin stripe of cloud extending backwards from the wing tips (wing tip trails).

In this case the wing tip trails are forming circles because the rotor blades of the helicopter rotate.

More pictures: 1 – 2 – 3 – 4

Authors: Claudia Hinz and Peter Krämer

Rings of Quételet

Rings of Quételet are caused by rays of light which had been scattered by particles of dirt on a window pane and then were reflected from the rear side of the pane, interfering with rays having been reflected from the front side of the pane and then having been scattered by the same dirt particles.

The light source itself forms a white circle being surrounded by colourful rings which are caused by interferences.

These rings always begin with a blue one right outside the white circle and end up with a red one on the other side, away from the circle, no matter if the secuence begins in the centre of the pane or on its rim.

Place : Fehraltorf, Switzerland
Time : 24 August 2011
Digital Camera : Canon EOS 450D
Exposure time : 1/2500 sec , f/9 ,Focus length 22mm, ISO 100

Author: Michael Großmann, Kämpfelbach, Germany

Colourful flies leg

Since a short time I ever and again go down into the microcosm, a very interesting and colourful world. A lot of optical phenomena in the sky can also be found in this little world. The origin of the colours are usually diffraction and interference. Actually it´s no wonder, that these tiny structures are able to bend the light. So it´s probably possible that the bristles of a flies leg get colourful. Well, I think, I should analyze the hairs on my legs, too.. .

The compound eye shows interesting effects as well and the wings anyway. Some flies have seemingly a gold foil between the eyes.

More pictures of my journey into the microcosm can be found here: – 1 – 2 – 3 – 4 – 5

Author: Sven Aulenberg, Saarburg, Germany

Goat Willow Seed Corona

Around noon on April 22, 2011 (Good Friday), I went to a hedge near Limburg (Hesse, Germany). As the weather was sunny and dry, some goat willows (Salix caprea) sent a large amount of seeds into the spring air. Together with the bright mid day sunshine, these caused some surprising effects. Often there was just one bright area around the sun (1-2-3), but from time to time a colourful corona appeared in the seeds.

Author: Gerrit Rudolph, Hesse, Germany

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

“Bubble Bow”

This picture actually shows just a few air bubbles frozen in a sheet of ice. But when watching it from a certain angle against the antisolar point, I discovered these strange colours. These can be expressed as a kind of “inverted rainbow”, as the circumstances under which it formed are exactly reversed to those of a normal rainbow. In a rainbow, the spherical object consists of water, but in this “bubble bow” the spherical object consists of air.

But there is a crucial difference: The ordinary main rainbow is based upon a double refraction of light and one inner reflection. But in case of this “bubble bow” the light is refracted four times: When it enters the ice, it gets refracted for the first time. The second time is when it enters the bubble. Then it is reflected once (or several times?) inside the bubble before it gets refracted a third time when leaving the bubble and entering the ice again. Finally, it gets refracted a fourth time when it leaves the ice. The “bubble bow” formed by this procedure has the same sequence of colours as a normal rainbow. But I do not know if it can ever be seen as a whole.

Photograph taken in Barsinghausen-Egestorf (Germany) on March 5, 2010, with a Canon EOS 1000d camera. More pictures are here.

Author: Reinhard Nitze, Barsinghausen, Germany