Double rainbows - and more

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  • A double rainbow over Rotorua, New Zealand, as Ireland take on Russia during the Rugby World Cup. Photo: David Davies/PA Wire/Press Association Images

    A dramatic rainbow frames the almost-complete Shard building in London on a showery late afternoon in August. Photo: Lewis Whyld/PA Wire/Press Association Images

    People in Bristol saw this striking sky and rainbow during a big storm in August 2010. Photo: Ben Birchall/PA Archive/Press Association Images

  • Double rainbows - and more
    07.10.2011 08:04

     

    With the sun’s elevation lowering and the probability of showery days increasing, autumn provides a lot of opportunities for viewing rainbows.

    They can of course be seen at any time of year but not necessarily at any time of day. During summer it would need to be early morning or during the evening to have the sun at a low enough angle, unless viewed from a hill top or an aircraft.

    They always require sunlight shining through droplets of water, which might be rain but could also be from a waterfall or fountain. Light is refracted as it enters a water droplet, reflected from the far side and then refracted again as it exits back towards the watcher. A primary rainfall is always therefore seen with the sun behind the observer.

    Light rays fan out as through a prism; different wavelengths are refracted through different angles and we therefore see a spectrum of colours. The human eye perceives distinct banding but this is an illusion created by the way the brain process messages from its photoreceptors.

    Sometimes a fainter secondary rainbow can be seen above the primary bow. This is caused by a second reflection inside the water droplets, and as a consequence the order of colours is reversed.

    Far more rarely a tertiary or third-order rainbow can appear, when light is reflected a third time. These are hard to see because they are even fainter and turn up in an unexpected quarter of the sky. The further reflection means that they show up in the same half of the sky as the sun, the brightness of which also disguises there presence.

    Theoretically there could be any number of rainbows, and under laboratory conditions a 200th-order bow has reportedly been observed.

    Back in the real world nobody had reported anything more than a tertiary rainbow – until June this year, when Michael Theusner, a German photographer, spotted a fourth-order rainbow, although that only became clear once he’d processed his photographs.

    That meant that he was the first also to photograph a quaternary rainbow, and only the second even to capture a tertiary rainbow. The first of these was photographed only one month previously by Michael Grossmann, another sky photography specialist.

    Theusner spotted very similar conditions to those under which Grossmann had seen his third-order rainbow, and hurried into position. The fourth-order bow came as a very unexpected bonus.

    If only the myth about pots of gold were true he could have found an even larger bounty.

    By: Stephen Davenport