Recent strange weather conditions have led to a number of extraordinarily bright local rainbows. They contain extra colours inside the usual violet. There are as many as three additional bands: a narrow one of orange-yellow, a wider one the vivid green of nocellara olives and a narrow one of purple. These extra bands occupy about a third of the width of the rainbow itself. What am I seeing?
• It sounds as if your questioner is seeing a supernumerary rainbow. This occurs when raindrops are relatively small and evenly sized. Light reflected inside the droplet can interfere with that reflecting off the outside, producing light and dark bands – just like the classic double slit experiment showing that light can behave as waves or particles.
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Because different wavelengths of light are refracted by different degrees, the exact interference patterns vary according to colour. If the droplets are of even size, they all line up and produce more rainbows inside the main bow. The strange colours are probably caused by colours mixing where the interference patterns overlap. If you photograph a rainbow and turn up the contrast, you might see many supernumerary bows.
Stephen Jorgenson-Murray, Frankfurt, Germany
• This type of rainbow is known as a supernumerary rainbow. They are rare, so your questioner was lucky to see one. I saw one on 27 May 2016 at Great Shoddesden in Hampshire, UK. It is the only one I have noticed in 62 years of knowing what a rainbow is. I have enhanced the photo (left) to show the colours clearly.
The effect results from raindrops of a uniform size of 1 millimetre or less. It was recognised in the 19th century that this effect can’t be explained by optics alone (as ordinary rainbows can), and is evidence of the wave nature of light.
Brian Pollard, Launceston, Cornwall, UK
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