The question of whether indigo is considered one of the colors in the rainbow has been debated for centuries. While most people are familiar with the 7 colors of the rainbow – red, orange, yellow, green, blue, indigo, and violet – some argue that indigo should not actually be included as its own distinct color. This article will examine the evidence both for and against indigo being considered part of the rainbow spectrum.
The History of the 7 Rainbow Colors
The concept of 7 colors in the rainbow originated in the 17th century when Sir Isaac Newton identified the color spectrum produced when white light passes through a prism. Newton labeled the colors as red, orange, yellow, green, blue, indigo, and violet. His color assignments were influental at the time and established the popular rhyme children learn “Roy G. Biv” to remember the order of colors in the rainbow.
However, Newton’s color labels were subjective and not based on precise wavelengths of light. Over time as optical science advanced, some scientists began questioning Newton’s color labels, particlarly indigo which occupies a small slice of the visible color spectrum between wavelengths of about 445 and 425 nanometers.
The Argument Against Indigo
Those who argue against including indigo as a distinct rainbow color make several key points:
| – Indigo’s wavelength range in the spectrum is very narrow compared to other colors |
| – The human eye cannot easily distinguish indigo from blue and violet |
| – Modern color science finds no distinct separation between blue, indigo, and violet wavelengths |
| – Most cultures and languages historically did not recognize indigo as its own color, but rather classified it as a shade of blue or violet |
Essentially, opponents of indigo claim that Newton arbitrarily labeled part of the short wavelength end of the visible spectrum as indigo just to have 7 colors to match the 7 notes of the musical scale. Physically and perceptually, indigo does not stand out as obviously separate from blue and violet. For this reason, many argue only 6 distinct colors can clearly be identified in the rainbow – red, orange, yellow, green, blue, and violet.
The Case for Keeping Indigo
On the other side of the debate, supporters of indigo make counterarguments for why it deserves to remain defined as its own rainbow color:
| – Isaac Newton’s original color assignments including indigo have historical importance and familiarity. |
| – While small, the indigo portion of the spectrum does have its own distinct wavelength range. |
| – The 7 colors of the rainbow including indigo are widely taught to children and incorporated into popular culture. |
| – Some individuals claim to be able to perceive indigo as a distinct color in the rainbow. |
| – The rainbow color sequence sounds more harmonious with 7 colors rather than 6. |
To defenders of indigo, it does not need to occupy a large swath of the spectrum or be obviously distinct from neighboring colors to be considered valid. Its long historical association with the rainbow gives indigo a perceptual reality whether or not it is physiologically separable from blue and violet. Altering the traditional 7 rainbow colors would disrupt what is familiar to many people.
Scientific Analysis of Indigo
Modern scientific techniques have been applied in attempts to resolve the indigo debate by more objectively evaluating how humans perceive spectral colors:
| – Spectrophotometry shows indigo has a distinct wavelength peak between blue and violet, around 445 nm. |
| – Visual response studies find the eye has some differential response to indigo wavelengths compared to adjacent colors. |
| – Color matching experiments indicate people are able to distinguish indigo from blue and violet at least in certain situations. |
| – However, tests also show indigo is the most difficult spectral color for people to perceive as distinct. |
So from a scientific perspective, indigo appears to be a real but subtle color distinction that falls at the edge of human visual discrimination. This may explain the ongoing disagreement around indigo in the rainbow.
Indigo in Rainbows
Part of the challenge in determining whether indigo qualifies as a rainbow color stems from the transitory and variable nature of rainbows themselves:
| – Rainbows only appear under certain conditions and viewing angles. |
| – They display a continuous spectrum of color, without neatly divided bands. |
| – Color appearance changes with factors like a rainbow’s width and background. |
| – Rainbows display a mix of spectral colors from refraction and non-spectral colors from light scattering. |
Given these factors, people may perceive and describe the colors within a rainbow differently based on individual eyesight, psychology, and interpretation of the visual effects involved.
Indigo Dye and Pigments
Complicating the indigo debate is the fact that throughout history, the color term “indigo” has been used to describe not just a spectral wavelength but also distinct blue/violet pigments used in dyes and paints.
The original “indigo dye” came from plants in the genus Indigofera and created a distinctive blue/violet color historically prized across many cultures for textile dyeing and other uses. While this indigo dye color is similar to the indigo portion of the visible spectrum, it does not have an identical match. Modern synthetic pigments also continue to be classified as indigo for their resemblance to traditional indigo dye.
So when people refer to “indigo,” they may have in mind either the spectral indigo wavelength or the distinct indigo dye color that lacks a precise spectral equivalent. This can create ambiguity around what “indigo” means in the context of the rainbow.
Conclusion
The question of whether or not indigo can be considered one of the rainbow colors has no single definitive answer:
| – In terms of precise physics, indigo occupies its own narrow portion of the visible light spectrum between blue and violet. |
| – But perceptually, indigo is difficult for the human eye to distinguish as a separate color and often gets classified as a shade of blue or violet. |
| – Historically and culturally, indigo is traditionally recognized as the seventh rainbow color in many societies. |
| – Creative definitions of rainbows need not be confined solely to spectral colors, so indigo’s inclusion is a matter of convention not scientific mandate. |
Given these conflicting facets, the status of indigo ultimately remains a matter of interpretation and personal philosophy on what defines the rainbow’s sequence of color. Whether viewed as real or invented, indigo’s role in the rainbow continues to be debated. But most modern sources still present the traditional 7-color sequence including indigo as the prevalent way to list the bands of color in a rainbow.
