MOTION MOUNTAIN

126 3 what is light?
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F I G U R E 80 Exceptionally many supernumerary rainbows (© Denis Betsch).
the following sequence: 1. black and white, 2. red, 3. green and yellow, 4. blue, 5. brown,
6. mauve, pink, orange, grey and sometimes a twelfth term that differs from language to
language. (Colours that refer to objects, such as aubergine or sepia, or colours that are
not generally applicable, such as blond, are excluded in this discussion.)The precisediscovery
is the following: if a particular language has a word for any of these colours, then
it also has a word for all the preceding ones.The result also implies that people use these
basic colour classes even if their language doesnot have a word for each of them.These
strong statements have been confirmed for over 100 languages.
Fun with rainbows
The width of the usual, primary rainbow is2.25°, for the secondary rainbow it is about
twice that value (which is one reason why it is less bright). The width is larger than the
dispersion angle difference given in Figure 75 because the angular size of the sun, about
0.5°, has (roughly) to be added on top of the angle difference.
The finite size of droplets leads, via interference, to the supernumemrary rainbows,
as mentioned above. If the droplets are small and all of the same size, the number of
supernumerary rainbows increases, as Figure 80 shows strikingly.
If the droplets are extremely fine, the rainbow becomes white; it is then called a fogbow.
Such bows are also often seen from aeroplanes. If the droplets are not round, for
example due to strong wind, one can get a so-called irregular or twinned rainbow. An
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net