Somewhere under two Rainbows

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Somewhere under two Rainbows

A strange natural phenomenon was recently seen in Iceland. Two rainbows were seen at the same time. Hróbjartur Thorsteinsson explains this on the web of the Icelandic Met Office.


© Hrafnkell Þorsteinsson
Reflected rainbows.Friday July 30.2010, Sunset and still sea at the golfcourse Bakkakot in Mosfellsbær

Rainbows are formed when sunlight shines into raindrops and is reflected on the backside of the drops. Different wavelengths of light break at different angles and this causes the rainbow to be formed in different colors. Simple rainbows form a full circle with a 42° radius directly from the sun from the viewpoint of the spectator. Hence the center of the circle of the rainbow is below the horizon.

It is uncommon to see a full circled rainbow, because a lot of raindrops have to be below the horizon. However, it is possible from a great height, from high skyscrapers, airplanes or steep mountain slopes. Often a second rainbow with a radius of 50¨ is also visible with reversed colors. Such outer rainbows are explained by more than one reflection in raindrops.

Hence is odd to see two rainbows with the same order of colors that touch at the base as can be seen in the picture.

Rainbows that touch have different centers which can be explained by the existence of two sources of light. The most common explanation is reflection of sunlight from still water.

When the photo was taken it was absolutely calm. The farmhouse on the photo is close to the sea. The higher rainbow on the picture is therefore a reflected rainbow and has its center in the height of the sun. The two rainbows touch at the horizon, because the sources of light are equally distant from the horizon, above and below it.

For more on reflection reinbows click here.