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.

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

Strange Rainbow

Reinhard Nitze observed at Barsinghausen (Niedersachsen, Germany) on the morning of July 7, 2007 a strange rainbow produced by a passing shower. Owing to the low solar elevation it had a predominantly reddish colour. When he saw the photos he remarked on the strange step change in brightness, colour saturation, radius and width near the left-hand base of the bow (photo with unsharp mask). The change is marked on the photo by an arrow and is not an artefact because it is present on other images (1 2 3). The upper part of bow is considerably brighter and more strongly coloured although blues are weak and violet is completely absent. This is not easy to explain. A cloud might be shadowing the weaker part of the bow but that does not explain the width or radius change. Smaller drop sizes in the lower region might be responsible and could be obscuring rays from larger raindrops further away. Effects of reflected light bows can be ruled out because no water was nearby. Somewhat similar unusual rainbows were imaged by the Japanese observer Yuji Ayatsuka.

Purple sun?

These pictures were taken on a flight from Geneva to Warsaw on 18 July 2007. Shortly, after the sun disappeared behind a bank of clouds, I was surprised to notice that the LCD display of my Lumix TZ-1 camera was showing a purple sun. As purple is not a colour not normally associated with the sun, my first thought was that my camera had been damaged by taking pictures of the sunset. However, it soon became clear that the camera really could "see" the sun - despite the fact that the sun was completely invisible to my eyes! In fact, the sun remained visible to the camera for about 2 minutes after it disappeared according to my eyes.

Notice that the sun appears to have been squashed vertically (due to atmospheric refraction).

The explanation for this phenomenon is that the CCD sensors used in digital cameras have their peak sensitivity in the infra-red - typically at a wavelength of about 1000 nm, which is well beyond visible spectrum of 400 - 700 nm. You can test the infra-red performance of your own digital camera by pointing a TV remote control at the lens of the camera from a distance of about 15 cm (6 inches). Most remote controls transmit infra-red at wavelengths of 850 - 1000 nm. Your camera viewfinder will probably show a purple light when you press a button on the remote control. The purple colour suggests that the red and blue sensors in the camera are sensitive to infra-red - but not the green sensor. Hence, the purple sun .....

Algal optics

The season for algal optics has started, the first display was seen here in Finland already in the beginning of April. No photos was taken, but a gallery from my work on this stuff in last summer is here.

The algal films that display optical phenomena are clearly not as rare as have been thought. In the Baltic sea rocky islands about every 10th freshwater pool came with algal optics last summer. In Bulgaria, the species Chromophyton rosanoffii, which is responsible for the phenomena in the photo, has been described as common.

When it has not been raining for some days, go look for any freshwater ponds, puddles and pools. If the water surface is covered by a thin film, chances are that some sort of optical phenomena is visible in the sun light.

Flag corona

Light diffraction doesn't only originate from aerosols like little water droplets or pollen floating freely in the atmosphere, but also from so-called diffraction gratings. These consist of a large number of equally spaced holes or slits, from which the light rays interfere and form an interference pattern. In this example, which I photographed in the beginning of April, the thick woven fabric of the European flag serves as a diffraction grating and shows a beautiful corona.

Green and Blue Flash

On December 26th I climbed the 1000m high Plettenberg to observe and photograph the setting sun with 1m focal length to look for the green flash. The transparent air and an inversion layer were promising, however a few clouds with their top at approximately the same altitude were disturbing. The upper limb of the sun turned out to be quite turbulent showing green rims and flashes, but also some blueish apparitions.
Why a blue flash?
A green rim of the setting or rising sun occurs due to differential refraction in the atmosphere. If conditions are extremely clear, also the blue light has a chance to get through, and there might be even a blueish rim. With a temperature inversion layer in the atmosphere, upper segments of the solar image might get separated from the rest of the solar disc. In the final moments of these elusive segments they do appear green and sometimes even blue (green or blue flash). However, it is not clear to me why in my observations both happens. Most last moments are green, whereas the third frame shows a blue color and at the same time other turbulent segments with a green color. Should not every segment turn from green to blue, at least in the very last visible moment?
Maybe something more than just clear air plays a role for a blue flash visibility!?
It is the first time I see green and blue flashes simultaneously in one image. I strongly encourage other observers to record video data to show these effects in higher time resolution.

See large image with blue flash
Image sequence with description

Colours in the airplane window

While flying from Izmir to Ankara in Turkey on monday the 2nd of April 2007 at 10.15am Selen Ediger wanted to take an aerial shot of the mountains below with my canon ef 10-22mm lens and Hoya multi coated polarizing filter. With naked eye the colors were not visible but when she looked through the camera she saw that the land and the sky was covered with rainbow colors. The contrast and the saturation are adjusted a bit.

What's the origin of this colours?

Reflected rainbows near Spitsbergen

On 22nd August 2006 between 20.35-20.50 hrs, while cruising on the Isfjord near Spitsbergen, Nicola Boll watched these wonderful reflected rainbows. The lower bow is an ordinary primary bow. Intersecting it and curving upwards above it is a 'reflection bow'. The bow is formed by sunlight reflected off the smooth waters of the fjord. The upward going reflected rays are, in effect, from a 'virtual sun' the same distance below the horizon as the real sun was above it. Since rainbows are always centred opposite the light source, the centre of the reflected bow is up in the sky and the bow correspondingly high. In the picture enhanced by unsharp masking a reflection bow is also visible curving upwards above and to the right of the secondary rainbow.

Rainbow season started early with a twinned primary

While homeward bound from work a small rainshower started to pour in bright sunlight and I looked the the north and got a flat looking rainbow. I was ride-sharing when this happened and we pulled over and I got some photos and the bow looked unusual. Upon downloading the photos I applied unsharp mask and noticed the primary was twinned primary bow. This is the first rainbow for me this year. I would of expected to see one in the spring not February!