As children we learn that the rainbow has seven colours: red, orange, yellow, green, blue, indigo, violet. We consider it a fact of nature, almost like the number of days in the week. In reality this division has a precise history and a surprising origin, which weaves together the birth of modern physics and a very ancient idea: that of a universe ordered according to musical harmony.
The prism experiment
In the second half of the seventeenth century Isaac Newton carried out a series of experiments with prisms. Letting a ray of sunlight pass through a hole in a darkened room and then through a glass prism, he observed that the light spread into an elongated band of colours: the spectrum.
The decisive step was to show that the prism did not «create» the colours, but separated components already present in white light. With a second prism Newton isolated a single colour and showed that it did not break down further; with another arrangement he recombined the colours into white light. The results were set out systematically in the work Opticks, published in 1704.
A continuous spectrum
The spectrum of light is a continuum: the colours shade gradually into one another, without sharp boundaries. Dividing it into parts is a choice. In his early writings Newton spoke of five main colours: red, yellow, green, blue, violet. Later he added two, orange and indigo, reaching seven.
Why seven? The answer does not come from physics.
The analogy with music
Newton explicitly compared the division of the spectrum to the division of the musical scale, which in the Western tradition counts seven different notes before returning to the octave. He tried to match the widths of the colour bands to the intervals of the scale, convinced that a deep analogy existed between the harmony of sounds and the harmony of colours.
He also arranged the colours in a circle, joining red to violet, just as the notes close upon the octave. It was one of the first colour circles in history.
This choice reveals an often forgotten side of Newton: beside the mathematical physicist there was a thinker deeply interested in the hidden harmonies of the cosmos, in theology and even in alchemy, to which he devoted many writings that long remained unpublished.
The ancient harmony of the spheres
The idea that the cosmos is ordered according to musical proportions goes back to antiquity. Tradition attributes it to Pythagoras and his school, who are said to have discovered the simple numerical ratios between string lengths and musical intervals. From this arose the doctrine of the harmony of the spheres: the planets, as they move, would produce sounds ordered into a cosmic music that the human ear does not perceive.
In the Middle Ages and the Renaissance this idea was taken up by many authors. Kepler, shortly before Newton, wrote a work on the harmony of the world in which he sought musical ratios in planetary motions.
The number seven
Seven also carries enormous symbolic weight in many traditions:
| Field | Example of a sevenfold set |
|---|---|
| Ancient astronomy | seven planets visible to the naked eye |
| Calendar | seven days of the week |
| Music | seven notes of the diatonic scale |
| Bible | seven days of creation |
| Newtonian optics | seven colours of the spectrum |
We do not know how aware Newton was of all these resonances, but his choice fits into a long tradition that sees seven as the number of ordered completeness.
Indigo, a disputed colour
Indigo is still the most debated of the seven colours. Many observers struggle to distinguish it from blue and violet, and some scholars believe Newton included it mainly to complete the sevenfold scheme. It is an eloquent example of how theory can guide the eye: we see seven colours partly because we have been taught there are seven.
A symbolic reading
The story of the seven colours shows that science and symbol are not always separate. One of the founders of modern physics organised light according to a musical and numerical model inherited from antiquity. For those who study tradition it is a valuable example: the intuition of a harmonic order in the world also accompanied the birth of experimental thought.
In short
- Newton showed with prisms that white light contains the colours of the spectrum.
- The spectrum is continuous; the division into seven colours is a choice.
- Newton added orange and indigo by analogy with the seven notes of the scale.
- His idea recalls the ancient Pythagorean doctrine of the harmony of the spheres.
- Indigo, hard to distinguish, reveals how much theory guides the eye.
