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Can you ever see a complete circular rainbow?

Can you ever see a complete circular rainbow?
David Gray from Dorset (Age 45-54)

Why are full rainbows always semi-circular?
Victoria Ferguson from Stockton on Tees  (Aged 5-14)

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2 Responses

  1. Hi Young Scientist,

    No – a complete circular rainbow is not visible from one point because the angle to which an observer’s eye is turned above (or away from) the ‘direct line to the centre of the Sun’, which must be behind their head, is crucial to the phenomenon.

    In fact, it is required that this line of sight must be just over 40 degrees above the ‘axis’ of the Sun’s light in order to view a ‘Primary Rainbow’. Most people are then able to see 7 colours in the sky starting with violet then indigo, blue, green, yellow, orange and at the top, red. A Secondary Rainbow is sometimes visible higher than the Primary Rainbow. This ‘Secondary Rainbow’ begins at an inclination of 52 degrees with the colour red progressing higher through the spectrum to 54.5 degrees with the colour violet. This reversal in the order is caused by a second reflection of the light within the raindrop before it emerges. The colours are not sharply separated but are really parts of a continuous band of colour which reveals the different energy frequencies in visible ‘white light’. Another interesting point which may be noticed is that the rainbow colours appear to be brighter nearer to the horizon ( where the ‘ends’ seem to touch the ground) . This effect is produced by the greater focussing power of the larger raindrops becoming stretched and more flattened spheres as they near the ground. Rainbows are fascinating to see and can seem to go on forever in a continuous circle if followed, as that crucial angle of just over 40 degrees to the axis of the sunlight ( from behind remember ) is maintained.

    I hope this answers your question and stimulates your interest further to find out more about light and the bending ‘refracting’ or bouncing ‘reflecting’ of it.

    Good Luck, Rockno3

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