The colors of the rainbow are a natural phenomenon that has captivated humans for millennia. The sequence of colors – red, orange, yellow, green, blue, indigo, and violet – is etched in our minds from an early age. However, most people don’t realize that the number of colors in the rainbow hasn’t always been 7. The color indigo was once removed from the rainbow by scientists and philosophers, only to be later reinstated. This article will explore the history behind the changing rainbow, and why indigo has had such a turbulent relationship with the other colors.
The Original 7 Colors of the Rainbow
The concept of 7 colors in the rainbow originated in Ancient Greece, where philosophers like Aristotle and later Isaac Newton assigned 7 colors to the rainbow based on the 7 notes of the musical scale. The colors were believed to be: red, orange, yellow, green, blue, indigo, and violet.
This 7 color sequence would come to be known as the ROYGBIV sequence. The ROYGBIV acronym stood for the first letter of each color – Red, Orange, Yellow, Green, Blue, Indigo, Violet. Each color blended into the next seamlessly, creating the rainbow spectrum.
For centuries, this 7 color sequence was the undisputed representation of the rainbow for scientists and artists alike. The color indigo – named after the tropical plant that produced the dyestuff – occupied its place between blue and violet along the rainbow.
The Omission of Indigo from the Rainbow
In the 18th and 19th century, scientists began taking a more empirical approach to understanding the spectrum of light that caused the rainbow. They started questioning if the ROYGBIV sequence really reflected the true order of rainbow colors.
In particular, many began to doubt the distinction between indigo and blue. The wavelengths of light that corresponded to these colors were very close together, making them difficult to discriminate. To the naked eye, the indigo band in a rainbow was weak and not always discernible from blue.
| Color | Wavelength (nm) |
|---|---|
| Red | 700 |
| Orange | 620 |
| Yellow | 580 |
| Green | 530 |
| Blue | 470 |
| Indigo | 450 |
| Violet | 400 |
In 1802, English chemist Thomas Young presented a paper to the Royal Society in London describing experiments that measured the wavelengths of light corresponding to each color. He concluded that indigo did not merit differentiation from blue.
Other prominent scientists like James Clerk Maxwell and Hermann von Helmholtz came to similar conclusions that indigo should be omitted from the rainbow. They often began referring to the primary colors of the rainbow as ROYGBV – removing the I for indigo.
By the mid-19th century, many textbooks and color societies had shifted to describing the rainbow as having 6 primary colors rather than 7, relegating indigo to a minor color between blue and violet.
Goethe and the Revival of Indigo
While scientists were discarding indigo, an unlikely advocate was fighting for its restoration – the German writer and statesman Johann Wolfgang von Goethe.
Goethe was fascinated by color and became convinced that the traditional 7 color sequence was correct. In his 1810 book Theory of Colours, he provided an aesthetic justification for keeping indigo distinct from blue:
“The blue of the sky reveals to us the basic law of chromatics…Let us then not look upon the sky simply as blue, but as darkness modified by light, appearing blue according to its depths and black according to its heights…In its blue it tends to violet; in its blackness it tends to yellow…The bluish upper sky tends to violet, the darker lower sky to green and yellow. Out of these two colors, which are so near akin to blue, the bluish green and the bluish violet, the pure blue is formed in the middle heights of the sky.”
Goethe argued that the gradations between blue, indigo, and violet were perceptible, and therefore justified keeping indigo as a distinct rainbow color. His views helped revive support for indigo among artists and philosophers, who preferred the more romantic 7 color rainbow.
Modern Recognition of Indigo
By the early 20th century, indigo had regained its place alongside the other 6 colors of the rainbow, even if its distinction from blue remained debatable among scientists.
Part of its persistence can be credited to the influence of Sir Isaac Newton. As the discoverer of the visible light spectrum, Newton’s ROYGBIV sequence carried enormous weight. Out of reverence for Newton’s observations, the 7 color description endured.
Later scientific advancements like spectrophotometry would uphold indigo as a color with distinct borders between 450-420 nm in the EM spectrum, sandwiched neatly between blue and violet.
While the wavelengths of indigo and blue are close, improved technology showed that indigo photons do interact differently with our eyes than blue ones, justifying its separation. We now know:
| Color | Wavelength | Frequency (THz) |
|---|---|---|
| Blue | 450–495 nm | 606–668 THz |
| Indigo | 420–450 nm | 668–789 THz |
Modern color standards like sRGB recognize indigo as a distinct color occupying its own place in color wheel models.
While some contention remains among visual scientists, indigo is now firmly re-established as one of the 7 main constituent colors of the rainbow.
Why Removing Indigo Was Controversial
The removal of indigo from the rainbow in the 18th-19th centuries was controversial for several reasons:
- It disrupted a long-held tradition dating back millennia.
- It was perceived by some as an assault on metaphysical views of color and light.
- It undermined Isaac Newton’s authority and credibility.
- It ruined the memorability and symmetry of the “ROYGBIV” acronym.
- It was hard for people to disregard subtle indigo tones they perceived in the rainbow.
Altering the sequence of rainbow colors required overturning assumptions, conventions, and visual evidence that were deeply ingrained over centuries. The distinction between indigo and violet was subtle, but many still believed it was perceptible. Removing indigo seemed an unnecessary disruption.
The revival of indigo reminded us that while rainbow color divisions are based on science, there is still art, psychology, and philosophy involved in how we name, categorize, and appreciate color. In the end, the human tendency to prefer memorable patterns and traditions prevailed to resurrect indigo against scientific objections.
Conclusions
While indigo’s distinction from blue remains tenuous from a strictly scientific view, aesthetic and cultural factors contributed to its resurrection as one of the 7 main rainbow colors. Indigo’s story illustrates that the rainbow spectrum is not defined entirely by physics – human perceptions, traditions, and biases also shape our color categorization systems. The context-dependent omission and reinstatement of indigo shows that resolving ambiguities at the borders between spectral colors remains an ongoing process, informed both by science and human experience.
