The colors of the rainbow have long fascinated humans, appearing in art and literature throughout history. But the exact number and names of the rainbow’s colors have been a source of debate. While most people can easily name the colors red, orange, yellow, green, blue, and purple, the status of the seventh color, indigo, is less clear. Is indigo truly an official part of the rainbow spectrum? Or is it an artifact of history that has refused to fade away? Here we will examine the evidence surrounding indigo’s rainbow status.
The History of the 7 Rainbow Colors
The first known description identifying 7 colors of the rainbow appears in the early 17th century work De Iride Et Coloribus (“On the Rainbow and Colors”) by French mathematician and philosopher René Descartes. Descartes identified the colors as red, orange, yellow, green, blue, indigo, and violet. His scheme matched the 7 notes of the musical scale, and became widely adopted.
In 1672, English physicist Isaac Newton also published a description of the rainbow featuring 7 colors. Newton was influential in solidifying indigo as a color between blue and violet on the spectrum. However, Newton did note that the transitions between spectral colors are not sharply defined.
The popular “Roy G. Biv” mnemonic device for recalling the 7 rainbow colors – Red, Orange, Yellow, Green, Blue, Indigo, Violet – was coined in the early 19th century. This helped further cement the 7 color scheme in culture and education.
Why Indigo Was Included
There were several factors that motivated early scientists like Descartes and Newton to include indigo as a distinct spectral color:
- The influence of the 7 note musical scale. Identifying 7 rainbow colors created an appealing parallel.
- The desire to classify blue and violet as separate colors. Having indigo between them helped accomplish this.
- The fact that humans can perceive indigo as a distinct hue when saturated monochromatic light at around 445 nm wavelength is viewed.
- The existence of certain blue dyes and pigments that appear to have a slightly different hue than pure blue. These were classified as “indigo.”
Additionally, the properties of the indigo plant from which the indigo dye was derived fascinated early scientists. This intriguing botanical connection likely also helped spur the inclusion of indigo as a spectral color.
Controversy Around Indigo
Despite its long history of inclusion, indigo’s status as a proper rainbow color has been regularly challenged by critics. Some key objections include:
- The human eye cannot discriminate indigo as a separate spectral color under normal viewing conditions. The distinction only appears with abnormally saturated monochromatic light.
- There is no precise wavelength range that defines indigo. Proposed ranges (about 445 to 425 nm) overlap with blue and violet.
- Modern color scientists do not recognize indigo as a separate spectrum color. Blue and violet alone are considered adequate.
- Cultural associations and not science are largely responsible for indigo’s continued inclusion on the list of rainbow colors.
These objections have led many experts to conclude that indigo should not be regarded as a true spectral color. Nonetheless, indigo remains engrained in popular culture and education as one of the main rainbow colors.
Evidence For and Against Indigo
| Evidence Supporting Indigo | Evidence Against Indigo |
|---|---|
|
|
Modern Definitions of the Rainbow
Among current scientists, it is generally accepted that the visible spectrum contains 6 major color bands: red, orange, yellow, green, blue, and violet. Indigo is not recognized as scientifically distinct from blue and violet.
However, the popular conception of the rainbow retains the 7 color schema. Major meteorological and educational organizations continue to endorse the traditional rainbow color sequence of ROYGBIV.
For example, the World Meteorological Organization publishes the following official definition:
The rainbow is a continuum with primary colors of red, orange, yellow, green, blue, indigo and violet.
Many children’s books and educational materials also still teach the 7 rainbow colors including indigo.
Conclusion
While modern scientists do not consider indigo a truly separate spectral color, it remains ingrained in culture and education as one of the main hues of the rainbow. Indigo’s inclusion appears to originate more from musical associations, botany, and aesthetics rather than science. But after centuries of tradition, indigo’s status as a rainbow color seems unlikely to change, whatever the modern technical analysis may be.
So in summary: while not scientifically sound, indigo can still be considered an “official” rainbow color due to its widespread cultural adoption and persistence over time. The rainbow is a phenomenon rooted in both optical science and artistic tradition. With indigo as the case demonstrates, cultural factors are equally key to determining the rainbow’s perceived sequence of color.