Reflected Sunlight Dewbow
On August 19, 2010, Jérémie Gaillard made an interesting discovery when looking at the surface of the lake Etang de l´Alleu which is located in the French community of Saint-Arnoult-en-Yvelines. The water was covered with pollen, on which droplets of dew had formed. In these droplets two colourful rainbows were visible. Dewbows can be understood as the lower part of a rainbow projected onto a horizontal plane. When a dewbow is fully developed, a semi-circle which opens towards the sides should be visible, the apex of which is situated at the lower end of the observer´s shadow. Equivalent to normal rainbows, primary and secondary dewbow should run parallely, but in Jérémie Gaillard´s observation they did not.
Instead, the second colourful bow fragment is a reflected sunlight dewbow. The surface of the water acts as a large mirror reflecting the sun. The reflected image of the sun now acts as a second source of light, which is situated as far below the horizon as the sun is above it. (angle of incidence = emergent angle). So the antisolar point for the reflection of the sun is above the horizon. This reflected antisolar point, which is located the double of the real sun´s elevation above the antisolar point, is the centre of the two rainbow circles for the reflected sunlight. So the additional rainbows are displaced upwards by the double sun elevation compared to the primary and secondary rainbow, making a rather unfamiliar appearance in the open nature.
Author: Claudia Hinz
Dew bows are a kind of rainbows. The difference lies in the plane of projection and in the fact that static droplets are reflecting the refracted light back into the eyes of the observer. The rainbow cone resulting, the apex of which is the oberver´s eye, is cut by the plane (the field). The result is a hyperbola, but for our eyes, there is always a circle!.
The first nights in October were rather cold, so that a lot of dew could form in the fields.
I knew that the following days were ideal for looking for the dew bow.
The main problem while observing dew bows is the brightness of the field. A polarization filter makes the dew bow contrast better from the background. (2)
An even better idea was filming the dew bow while driving along the farm track. This makes it contrast even more clearly.
Place : Neulingen, Germany
Time : 02 October 2011
DSLR Camera : Canon EOS 450d
Exposure : 1/25 sec, f/10mm, F/7, ISO 100
Author: Michael Großmann, Kämpfelbach, Germany
On June 25, 2010, Rüdiger Manig observed a double dew bow on unevenly spread morning dew on a leaf in Neuhaus am Rennweg (Thuringia, Germany).
Especially when he slightly defocussed his camera, the bow could be distinguished better.(1–2–3)
Actually, the distance between the two bows was less than 10°, the angle which one could expect in a double dew bow. Maybe, however, that the angle of refraction was significantly reduced by the deformation of the droplets on the leaf.
The photo above showing a sprightly dew bow was captured in a moist field crop at Kämpfelbach Germany on the night October 22, 2010. Since the photo was taken at night, the illuminating source is the almost full moon which is directly opposite of the dew bow at the anti-lunar point. The mechanics of a dew bow are similar to that of a rainbow. Moonlight is refracted and reflected within the dew drops. The city lights in the background are Karlsruhe.
Photo details: Canon EOS 450D camera; F/4; focal length 8 mm; ISO 100; exposure time 30 seconds; 3 photos stitched together.
Posted by Michael Großmann
Reflected light dewbow on water surface
I and my wife Eliisa Piikki got a hint about a light phenomenon on a lake nearby. Eliisa took some photos and there was a dewbow and a reflected light dewbow in those photos. They were formed on the waterdroplets that were settled down on the Chrysomyxa ledi needle rust (Small-spored spruce-Labrador-tea rust).
That rust can’t be seen in Finland every year, but this year it is very common especially in the Eastern and in the Northern Finland. Because of the rust the young needles of some spruces are brown.
Author: Jari Piiki, Finland
Heiligenschein and dewbow in moon light
Well, I wanted to observe moon halos that evening, but the sky was totally clear without any cirrus clouds.
I noticed that some blades of the plants on a field were rather wet. I walked some steps into the field and immediately noticed a clear brightening around the shadow of my head. I did a few steps to the left and to the right and could see a very faint dew bow. So I took some photographs at different exposure times and ISOs. The best photographs (1 2) I achieved using stop 4, an 18-mm-lens and at a time of exposure of 45 seconds at ISO 400.
Author: Michael Großmann, Kämpfelbach, Germany