Pedant's Revolt Fallacies Page

Excerpt from:

Chapter 11 - Science and Nature

Indigo, the sixth colour of the rainbow, was added to make up the numbers

Have you ever wondered why there are seven colours in the light spectrum, affectionately known as the rainbow, and only six colours in the artist’s colour spectrum? In Understanding Color, Linda Holtzschue points out that eighteenth-century German writer, poet and scholar Johann Wolfgang von Goethe’s six-hue spectrum ‘remains the convention of the artists’ whereas ‘Newton's seven-hue model …remains the scientist’s (physical) spectrum.’ She theorizes that ‘because the students who pursue sciences are rarely the same ones who go into the visual arts, the differences between the two ideas …generally go unnoticed.’

The extra colour in the light spectrum is indigo, which Holtzschue claims many people can’t distinguish as a separate colour. This may be because, as education lecturer Steve Farrow, claims in the Really Useful Science Book, ‘there are actually only six colours in the well-known rainbow of light.’

Goethe’s symmetrical colour wheel stands up to scrutiny. Farrow describes how the three primary colours, red, yellow and blue, mix in pairs to form the three secondary colours, orange, green and purple and ‘these six colours are present in the rainbow.’ So how did blue become blue and indigo in the light spectrum?

Seventeenth-century English physicist and mathematician Sir Isaac Newton split sunlight through a prism and named the resulting colours. ‘It is tempting’ writes Farrow, ‘to imagine that the great man “saw” seven colours in his rainbow.’ But since Newton was also an alchemist ‘seven was a lucky number.’ Holtzschue agrees: ‘Mysticism was a great part of Newton’s time.’ She theorizes that ‘he may have elected to include seven colours because of mystical properties associated with the number seven.’

Nineteenth Century Theories of Art (edited by Joshua C. Taylor) explains that ‘Newton saw seven colours in the prism, doubtless defined a poetic analogy with the seven notes of music; under the name of indigo, a seventh colour which is only a shade of blue... it is a licence that even the greatness of his genius cannot excuse.‘

In Introduction to Light, author Gary Waldman explains that 'more commonly today we only speak of six major divisions, leaving out indigo...[because]... a careful reading of Newton's work indicates that the colour he called indigo, we would normally call blue; his blue is then what we would name blue-green or cyan.' Waldman reveals that Newton derived the term 'spectra' for the colours he saw, from the word 'spectre' meaning ghost. The collection of colours he termed ‘a spectrum’.

Of course, colour-naming is subjective, and technically speaking every hue has its place in the spectrum. But if you find that you can’t make out the sixth colour in the rainbow, don’t worry, you’re not alone.