Glass Bead Bow on Tarmac

When passing a cross-walk, a colourful bow is not what one expects to see there. So Jordan Stabile was very surprised when discovering the colourful bow on the tarmac in Somerville, Massachusetts, USA.

The answer to this problem is a rainbow, but with the difference that in this case the light is not reflected by drops of water, but by transparent spherical glass pellets. These have a diameter of about 0,25 millimetres and are used for sandblasting especially at cross-walks. As glass refracts light in a bigger angle than water does, the bow is smaller than a normal rainbow, with a diameter of only 20°.

A glass bead bow can easily be reproduced by placing a lamp over a sheet of black paper, on which glass beads (available for example in building supplies stores) have been spread before. Then a colourful bow (photo) appears on the paper. As the distance to our two eyes is very small in this case, the bow can also seem to float tridimensionally over the paper.

Zero Order Glow

When rain is falling with the setting sun behind it, there sometimes appears a golden or deep red glow in shape of a semicircle around the sun. Literature calls this glow “Zero Order Glow”. The name means that this glow is a zero order rainbow. This is because the light is not reflected within the raindrops once or twice, as it is the case in primary and secondary rainbows. In the case of a zero order glow, the light passes through the raindrops without being reflected, leaving them only a bit deviated. So there is no bow-shaped concentration of light, and the zero order glow appears in form of a diffuse shining area around the sun.

In normal cases, the phenomenon is visible only above the horizon. But when this photographs (2 –3) was taken, a fine drizzle fell into the valley from a very low layer of clouds, causing also a glow beneath the horizon. Due to the low sun elevation and the long way the sunlight had to travel through the atmosphere, together with the additional light diffraction on the small drizzle particles, there is only the red light visible.

Author: Claudia Hinz, Wendelstein (1835m), 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

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 secondary bow, there is some evidence of “supernumary” colours beyond the violet endof the spectrum, as evidenced by pink colour of the reflections.

Author: Alan Clark

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