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Is the arc of any rainbow always the same?

Is the arc of any rainbow always the same?
Mary from Perth & Kinross (Age: 45-54)


One Response

  1. Hi Mary :).
    Yes! The arc of a rainbow which appears to be in the sky after rainfall is, to any observer, the same. However, to another observer nearby, ‘their view’ locates that rainbow in a slightly different position in the sky. Often this ‘Primary’ rainbow has a partner ‘Secondary’ rainbow just above it and with the colours in reversed sequence i.e. with red at the bottom. With the Sunlight coming from behind an observer, the ‘half angle’ of the cone of light reaching an eye is 42 degrees above the axis for the Primary Rainbow and 51degrees above the axis for the Secondary Rainbow. Rays of Sunlight are refracted then reflected (twice for the ‘Secondary Rainbow’) within the raindrops as they fall. Although they are naturally spherical, air currents and gravity tend to distort them (in particular the larger raindrops) into more flattened, fattened ‘oblate spheroids’ which could explain the alternative appearance of the emergent light rays. The rain drops which are outside the primary cone or inside the secondary cone can send no light to the observer’s eye and thus, to them, a darkened area called ‘Alexander’s Dark Band’ exists there. Whilst a continuous spectrum of colours is revealed, a quantised version is perceived which generally possesses the well known seven – red, orange, yellow, green, blue, indigo, violet. There are also several alternative shapes to the arc described above. Stripes, circles and flame-like bands may be seen and they each have individual angular appearances. Mist, spray – even from a garden hose, dew, fog and ice crystals can also produce the refraction required.
    I hope this summary helps but a great many more facts are known and many excellent photographs exist of these wonderful phenomena.
    Good Luck,

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