Why, when I see a rainbow, do I perceive two extra colours (yellowish and greenish) directly beneath violet? Am I seeing UV wavelengths, with my brain interpreting them as those colours?
Guy Cox
Sydney, Australia
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Your correspondent isn鈥檛 seeing in the ultraviolet range. Instead, they are seeing a supernumerary rainbow. This is caused by diffraction and interference after the initial refraction of sunlight by raindrops, and tends to happen when the drops are very small.
It is very difficult to photograph (believe me, I have tried).
Nick Canning
Coleraine, County Londonderry, UK
Rainbows sometimes show extra coloured bands directly below the violet band at the bottom.
Human eyes can鈥檛 detect the UV bow that exists below the violet band. However, the bands that your correspondent saw are visible light and are called supernumerary bows.
Their intensity and spacing vary with the size of the raindrops in the shower that creates the bow. Smaller drops give larger spacing of the colour bands, whereas more uniform drops give greater intensity of colours. These are the result of interference effects caused by the wave nature of light.
Here in Ireland, in summer, we get heavy afternoon showers and often see intense primary bows with strong supernumeraries.
The light from rainbows is polarised, so photographing them through a polarising filter greatly increases the contrast with the background sky.
Richard Swifte
Darmstadt, Germany
The extra colours are genuine. They appear because a rainbow in the sky isn鈥檛 a pure spectrum like that, for example, from a prism.
A prism has smooth, flat sides and is used with a point source of white light to produce the resulting dispersion of colours (equal to individual wavelengths), with each colour emitted in a specific direction. The water droplets that produce a rainbow have rounded sides, so light enters and leaves at various angles to the surface. Also, the sun isn鈥檛 a point source of light, but has an angular size of half a degree.
These factors lead to each dispersed colour being emitted over a wide range of directions, with consequent overlapping.
This smearing isn鈥檛 uniform, however: a particular colour is concentrated in a narrow band and is much weaker outside, and it is these bright bands that we see as the main rainbow arc.
They give a reasonable approximation to a pure spectrum, since the weak overlapping contributions from other colours aren鈥檛 obvious.
Below the main arc, there is a large, brightish, whitish area of all the less concentrated smeared colours, which, due to interference effects, contains faint bands of distinct colours. The least faint are those nearest to the main rainbow arc.
In good viewing conditions, a person with good eyesight could perceive some of these extra colours.
Pat French
Longdon Upon Tern, Shropshire, UK
We each different wavelengths of light at each end of the 鈥渘ormal鈥� visible spectrum, perhaps due to natural variations in the eye鈥檚 cornea or lens.
How we interpret wavelengths of light is cultural too. People who live in the Amazon rainforest perceive finer gradations of green than city dwellers, as demonstrated by physicist Helen Czerski in her TV series Colour.
Also, some of us can perceive UV light. I have had the lens in my right eye replaced with an artificial one. My right eye, but not my left, now perceives the longer UV wavelengths very brightly, which means I squint at the UV emitted by devices that kill flies.
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