Saturday, September 22, 2007

Shock wave refraction and iridescence over airplane wing

gnes' post with the beautiful colored contrail pictures made me remember to post the following photos. On a flight in a 747-400 from Beijing to San Francisco on August 18th I was seated right over the wing on port side of the aircraft. Initially I was disappointed with this window seat but I did get to see some interesting things.

The first of these was a condensation cloud over the wing. The airplane was still ascending around 20,000 ft, when we flew through (super)saturated air. The faster airflow over the top surface of the wing and associated drop in pressure caused condensation. The cloud showed beautiful iridescent colors. The water droplets in the cloud were so small at this stage and all so uniform in size that they diffracted the light with constructive interference over large angles.

On the second photo this condensation cloud extends behind the wing. This is because it takes energy for a water droplet to form in supersaturated air, but once the droplet forms it quickly grows (or it quickly evaporates if the air is not supersaturated, as in the first photo). In other words, when the air is supersaturated, all that is needed is a trigger to initiate droplet formation, after which growth occurs because the air is supersaturated.

This droplet growth is responsible for the colored trail between the exhaust contrails that can sometimes be seen, as in the photos posted earlier. But most often, the air is either not supersaturated, or already condensed into a cloud. Then, no long trail forms (only the engine contrails perhaps), or it is not visible (within cloud). Therefore it is not common to see this effect, especially from ground.

At times a huge and bright corona formed around the sun. In this photo the airplane was banking right and the sun appeared low over the wing. My apologies for the low quality photo - it is very hard to photograph through an airplane window with the sun in view.

The other interesting thing, which I had observed before, but only now could photograph well, is the vertical standing shock wave that sometimes can be seen dancing above the wing of a commercial jet. Here, because of the condensation cloud, this shock wave actually became visible. Usually it can only be seen due to the feeble refraction and miraging of structures on the wing (you won't notice this unless you look very carefully and at the right moment when the shock is positioned so that you are in its plane).

The normal shock occurs because air flowing over the wing has to flow faster than below it, and this flow can briefly reach the speed of sound even if the jet airplane is flying slower than the speed of sound. Because the Mach number is about 1, the shock wave is almost perfectly vertically oriented, normal to the airflow.

The shock plane in the photo can be seen as the vertical plane of enhanced condensation. I believe this is the rarefaction shock (as opposed to the compression shock) because condensation is enhanced behind the plane, indicating lower air pressure. Presumably the compression shock was somewhere ahead of this, invisible to me since I was not positioned in its plane.

The sharp density gradient at the shock wave causes a lateral mirage, but any miraging can only be seen if you are looking near grazing incidence along the shock plane (which is usually not perfectly planar but a little curved). The last two photos better show the miraging along the shock plane. Note the effect on the pylon of engine nr. 1.

Thursday, September 20, 2007

homemade reflected-light rainbow

Though these photos are old but when it come to making rainbows with garden hose sprays this is an unusual one. I knew reflected light rainbow required a smooth body of water to form and at the time we still had an above ground swimming pool and its surface was glass smooth and I got the garden hose turned on and saw not only primary and secondary but in between those was a reflected light primary! I continued to experiment and I got some photos with the old Kodak DC215 which has been retired since I use my Nikon D-100. I don't know if anyone has ever tried a homemade reflected light rainbow with a garden hose and a smooth surfaced swimming pool but mine may not have been the first or the last.

Contrail Colours

These two photos were taken by "Controll" and Mónika Landy-Gyebnár (Noli) in Hungary. The pictures show wonderful colour stripes in the contrails. The most fascinating thing is that the stripes have the airplane as their centre instead of the Sun, and as the plane was flying, the colours did not change depending on the angle to or the distance from the Sun.

The explanation of this phenomenon may lie in the varying consistency of the exhaust. The water vapour condenses, freezes, and the ice crystals become larger and larger. During this process, the refractive characteristics of the particles constantly change, and if the circumstances are ideal and the trail is not too thick, we might see such beautiful colour patterns. For the explanation, we owe thanks to “Uncinus”.

There are more photos available in this topic. See Controll's collection of colourful contrails and another picture taken by Mónika Gyebnár-Landy.

Wednesday, September 19, 2007

Spider Web colours

Thie picture of light reflected from spider webs, looking on a direction towards the Sun, with 2-stop underexposure and the camera out of focus. Note the fact that, although the web strand is out of focus in the direction at right angles to the strand, the colour bands are sharp in a direction along the strand.

Monday, September 17, 2007

Resin Bow Effects

These primary and secondary refraction effects from droplets of resin onspruce tree cones produce very intense colours, at 20 degrees and 90degrees from the Sun direction. The photos were taken out of focus and twostops under normal exposure to avoid colour saturation. In the secondarybow, there is some evidence of "supernumary" colours beyond the violet endof the spectrum, as evidenced by pink colour of the reflections.

Author: Alan Clark

Reflected-light Rainbow

This image was taken during a violent storm that produced golf-ball-sizedhail as it passed over. The interesting thing about this reflected-lightbow is that there was no large body of water to produce the reflection. The source of this reflection was most probably a wet highway, theTrans-Canada Highway, west of Calgary. Since this part of the highway wason a hill, the reflected-light bow appears relatively high compared to theexpected position from a horizontal reflection surface.

Author: Alan Clark

Saturday, September 15, 2007

Reflection of camera flash-light in a glass of white wine

A good friend of mine is celebrating his birthday early in September. Recently, we celebrated his 53rd birthday on a Saturday evening in an Indonesian restaurant in the centuries-old city of the town of Utrecht, in the central part of The Netherland. This (school)friend was one of the three guys who initiated a group of youngsters at the end of the sixties and the start of the seventies (of the 20th century), interested in astronomy, weather and ... optical phenomena. The initiative grew out to the nowadays Dutch "Vereniging voor Weerkunde en Klimatologie" and the "Halonet" optical observers group.
During the superb birthday dinner, one of the people of the restaurant made a picture with a digital camera-with-flash-light, while we were toasting for the health and wealth of the hero of the feast.
Absolutely by chance, it happened, the flash-light of the camera reflected in the glass of white wine I raised for the toast (the yellow "light" clearly visible in the picture).
It seems, some people are just born for optics ...

Wednesday, September 12, 2007

Mysterious colours in the thistle seed

Helga Schöps searches ardently for color dispersion effects in nature (like this close up of the banded diffraction pattern). Rainbows are not the only source. Helga photographed these recently at Hermsdorf, Thüringen, Germany. One is evidently a partial rainbow produced by dew drops on the fine hairy structure. The other colours in the thistle seed heads are more mysterious. Do you have an explanation?

posted by Claudia Hinz & Les Cowley

Tuesday, September 11, 2007

Reverse Lamp-Rainbow

On August 31st at 01:00 I took some long-time exposures of the Westerhever Lighthouse in Nordfriesland (Germany). It was raining a bit but this didn't matter because I wanted to display the rays of the lighthouse. Home again I reviewed the photos and was a bit surprised about a kind of arc, originating at a point in height of the lantern room and sloping downwards until it ends +/- horizontal (see pictures 1 2 3). I thought it could be a type of refraction phenomena but I couldn't explain to me what is was exactly. So I placed the pictures in the Meteoros-forum. Mark Vornhusen and Christian Fenn told me, that this arc is a type of rainbow called "reverse lamp-rainbow" and that these photos are probably the first displaying this phenomena. Both a 42 degree arc as well as a 51 degree arc are to be seen at the pictures.

The rainbows originates from the horizontal Lighthouse-born lightplain cutting the hull of the "Minnaert-cigar", an apple like shaped figure that describes all those points in which light coming from a source of light is reflectet in an angle of 42° respectively 51° to an Observer. In case of an usual source of light at every point of the Minneart-cigar a rainbow is being generated. But because of overlaying of these rainbows the colour-addition leads to a white light and no rainbow can be seen. However the thin light-layer of the lighthouse-beam only allows forming of rainbows at a small window of the minnaert-cigar and the rainbow becomes visible.

Author: Achim Christoph

Monday, September 10, 2007

Divergent light fogbow

Observed at Tuula (Estonia) on 10th September at 00:30. The fog condition was perfect at the time for the glory's rings merge into multiple supernumeraries. But the location was perfect as well which is surrounded by forest from east and west side generating the wind tunnel to blow the fog from the bog field in north or from the river in south. As long as I remember this location has been always very foggy and has been often flooded in spring-time. The light source I used was Johnlite-2940, which makes the car's headlights a joke.

I also observed a very bright and colourful glory and took some close-ups.

at 43mm. at 24mm at 30mm

Saturday, September 08, 2007

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.

Wednesday, September 05, 2007

Announcement of an eventual International Optical Meeting next year in The Hague The Netherlands

Dear fellow sky watchers,

at the moment, Peter-Paul Hattinga Verschure and I are making preparing efforts for an eventual International Optical Meeting, next year in The Netherlands. Our first aims are:

- to find a proper location for the meeting, and

- bring in sky watchers, interested in the proposed meeting, as many as possible

In this stage, I have contact with the staff of the organization of the yearly Eurasian festival "Pasar Malam Besar" (P.M.B.), here in The Hague. The staff of the P.M.B. is interested in our project, and is inclined to give hospitality to an eventual International Optical Meeting 2008 in The Hague.

If you want to learn more about the preparing efforts concerning the proposed meeting, please click on this link.

Thank you very much for your attention!

Frank Nieuwenhuys
The Hague, The Netherlands