Friday, August 22, 2008

Glass Bead Rainbow on Kitchen Table

The observation of a rainbow in glass beads on, respectively next to, a fresh route-indicator in a street I made by chance made me think about studying the effect also in the light of a street lamp. But unfortunately the glass beads had been scattered too much by the traffic within a few days. However, also by chance, I learned that glass beads are also used for facing the surfaces of, for example, metal objects, and so I asked our precision tool maker to bring me some...and then I started the experiment.

As a source of light I used a small bulb like those you find in a bicycle lamp, but without a reflector, because the light source should be as similar to a single point as possible. I scattered the glass beads on a black eloxated aluminum sheet (ca. 30 x 40 cm), and the result was overwhelming. Exactly as in the experiment made by Christian Fenn, the bow can be studied under different geometrical conditions when using a laser and a rotating mirror. The easiest way is to realize the reversed geometry just as otherwise the shadow of the head would be rather large as distances are small. Just put the metal sheet on a table, hold the lamp above it and look at it using different positions of your head. I also took some photographs after having attached the lamp to a mounting. This also shields the direct light. The first supernumerary is also visible, and like in the rainbow caused by water, a polarisation of light can also be proved. (1 2 3)

Seen through a microscope, the glass beads look like this:

I estimated the average radius to be at about 50 micrometers with an average variation of about 15 micrometers. But there are different sizes available. Similar to water drops of that size, the colours are rather blurred (this is especially obvious when you look at a glass bow in sunlight). The spectrum of light coming from a bulb is also rather "red" which causes the strange colours of the pictures.

As I was very fascinated by that phenomenon, I also calculated some simulations using the Airy-theory for glass beads (I could reuse some parts of the original text about the twinned bow for this). And in order to show the phenomenon from the observer´s position, I could use the text on halos on snow covers (so after 10 years the circle is closed...). An imaginary depiction seen from above obviously shows the "intersection through the apple", but as far as I know, nobody tried to explain the different width of the colour bands up to now. Tis effect becomes very obvious when "opening the inner bow by merging with the outer one" (This is very difficult to describe; you must have seen it). However, the geometrical data of the simulation are not exactly the same as in my observation because I did not execute any measurements while photographing.

Left: Seen from above with the observer´s position in the centre of the picture
Right: Gnomonical projection from the observer´s position (“simulated photograph”)

Author: Alexander Haußmann, Hörlitz, Germany

Tuesday, August 19, 2008

Yellow Rainshower

On August 13, I could witness another atmospheric phenomenon in Bochum, Germany.

In the evening, about half an hour before sunset, a rainshower approached from the west. Behind the shower, the sky cleared up very rapidly, so that the low evening sun could shine through the veil of rain. This caused very strange light conditions I never experienced before.

First there was a yellow squall line visible, followed by a bright yellow veil of precipitation. As it came nearer, another shelfcloud-like structure passed from south to east. This cloud was also strangely illuminated and showed orange and brownish colours.

The squall line in the western and northwestern part of the sky turned more and more orange, too, while the rain got an intense and bright yellow. The whole landscape was bathing in a strange and intense yellow light with orange-coloured clouds above and in the east.

When it began to rain, a rainbow appeared in the east, visible as a full semi-circle. The blue part of the bow had already gone, only red, yellow and a faint green were visible.

Then the sun sank beneath the horizon, and all colours faded away within a few minutes. So there was no really red rainbow that evening, but nevertheless it had been very impressive to me.

Author: Peter Krämer, Bochum, Germany

Twinned Rainbow

On August 13, after a rainshower had passed over Bochum, Germany, a twinned rainbow appeared. The bow was rather faint, but the twinning in its upper part was clearly visible. The red and the yellow colour bands of the rainbow were twinned.

On photographs of the rainbow, the twinning effect is hardly visible, but after oversaturating one of the pictures, the twinning turned out well.

During the rainshower, large drops were falling. A second shower about half an hour later with normal-sized drops also produced a rainbow. Even though this bow was very bright, it was not twinned, but showed a secondary bow and supernumeraries.

This is another hint for the theory to be right that the effect of twinned rainbows is really produced by large raindrops getting flattened by the resistance of the air when falling.

Author: Peter Krämer, Bochum, Germany

Friday, August 15, 2008

Light refraction on Fireweed-seeds

While testing my new Tamron AFR70/300 on several plants and flowers I passed a very steep slope overgrown with fireweeds.The sun was relatively low above the slope that I noticed a silky lustre beetween the seeds. As more I was moving the plants beewenn me and the sun, the effect merged into an incredible refraction of light on the silky like seeds. I shot a few pictures and those which where slighty off focus showed very bright colours. This effect seems to be similar with light refraction on spider-webs. In this case though colous where extremely bright.

More pictures: 1 2 3 4 5

Author: Rolf Kohl, Germany

Saturday, August 02, 2008

Summer twilight rays

Bright twilight rays on 16 July, 2008 in Green Bay, Wisconsin. They stretched nearly all the way across the peaceful evening sky. This photo was taken 20 minutes after sunset. I enjoyed the view from our front deck for about 10 more minutes.