I have seen many double rainbows, but can you get triple or quadruple ones? If so, where are the best places to see them? What is the maximum number that could occur at the same time?
Len Winokur, Leeds, UK
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We mostly only see rainbows when the sun is behind us. The reason is that light from the sun is internally reflected by the back of raindrops to re-emerge towards us. As it enters and leaves the drop, the white light is split into colours. Redder wavelengths emerge at wider angles than bluer ones, hence why red always appears as the outermost band of the primary bow and violet as the innermost.
Not all the original light escapes first time round, however. Some of it undergoes a second internal reflection before re-emerging. This has three consequences. Firstly, the secondary bow’s colours now emerge at wider angles than those of the primary bow, so the second rainbow always appears above the first. Secondly, because it is formed by less light, the secondary bow is fainter. Thirdly, the colour sequence of the secondary bow is flipped, with red now being the innermost band.
Further rainbows do occur, but they are so faint that have these been . The caveat with the third and fourth bows is that they would be behind you, so seeing them would require facing the sun, and they would probably get lost in the glare.
From ground level, we never see a bow’s full circle because the droplets projecting its lower section are below the horizon. But can be seen from the air.
Hugh Pumphrey, University of Edinburgh, UK
The n-th rainbow is caused by light reflecting n times from the inside of raindrops before escaping. Until recently, there were a variety of claims to have observed a third bow, but few were credible. In the past decade, photographs of and bows have been taken, emerging when the contrast in the photograph is very strongly enhanced.
Mike Follows, Sutton Coldfield, West Midlands, UK
The use of laser light has revealed rainbows up to the , but nothing beats seeing rainbows in the wild.
Peter Bursztyn, Barrie, Ontario, Canada
Multiple rainbows are best seen in bright sunlight when the air is clear and free of dust. I saw nearly complete arcs of three rainbows in the Great Rift valley in Kenya about half a century ago. The conditions were perfect: a band of rain had cleansed the air of dust, the sun was low on the horizon yet still very bright and dark rain clouds formed the rainbows’ background.
I have seen small portions of a third rainbow in various other places, but these never matched the nearly complete triple arc that I was treated to so many years ago.
Geoff Piltz, Eskdalemuir, Dumfries and Galloway, UK
I have seen a quadruple rainbow on the waterfront at Liverpool, UK. There was a double rainbow with a taller pair of rainbows above it. This second set started at the same apparent origins as the first set, but extended higher into the sky. After consideration, I have come to the conclusion that the first set was created by light directly from the sun, whereas the second set was made by light reflected off the river Mersey.
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