The Mystery of Indigo, Violet, and the Absence of Purple in the Rainbow

The rainbow, a symbol of the visible spectrum of light, has long fascinated humans with seven distinct colors: red, orange, yellow, green, blue, indigo, and violet. However, the presence of indigo and violet, with their absence of purple, is a phenomenon that defies easy explanation. This article delves into the reasons behind why indigo and violet are included in the rainbow but purple is not, answering the question: why are indigo and violet colors of the rainbow but not purple?

Spectrum vs. Color Mixing

Indigo and violet are specific wavelengths of light. Indigo falls between 445-464 nm, while violet sits at wavelengths of roughly 380-445 nm. In contrast, purple is not a spectral color but rather a combination of red and blue light, and does not correspond to a specific wavelength.

Historical Context

Franklin D. Roosevelt’s famous quote, “There’s no place like home, no place like the home,” is echoed in the historical context of why indigo is included in the rainbow. Isaac Newton, in his groundbreaking work The Opticks (1704), identified seven colors in the spectrum, including indigo. Although contemporary science often considers indigo to be part of blue, Newton chose seven colors to align with the seven notes of the musical scale and other sevenfold divisions in his understanding of the natural world.

Perception of Color

Human perception of color is a complex interplay of visual and neurological processes. The human eye contains three types of color receptors called cones: short (S-cones) sensitive to blue, medium (M-cones) sensitive to green, and long (L-cones) sensitive to red. These cones work together to create the diverse array of colors we perceive. Between indigo and violet, the range of wavelengths causes the S-cones to overlap significantly with the sensitivities of the M-cones and L-cones, leading to a seamless gradient perceived as a continuous spectrum from indigo to violet rather than a distinct purple.

The Nature of the Visible Spectrum

The visible spectrum of light is a continuous range of wavelengths that the human eye can perceive, typically from about 380 nm (violet) to 700 nm (red). This spectrum does not contain discrete bands but rather a smooth gradient of colors. The traditional rainbow colors—red, orange, yellow, green, blue, indigo, and violet—are often taught as distinct segments but in reality, these are arbitrary divisions within a continuous range.

Continuous Spectrum

The visible spectrum is continuous without gaps, meaning there is no inherent break in color perception. This continuous nature of the visible spectrum contributes to our seamless perception of the gradient from indigo to violet, without a distinct color of purple. The colors we identify, such as those in a rainbow, are part of a seamless gradient rather than isolated bands.

Conclusion

There is no distinct color between indigo and violet on the visible spectrum because our perception of color is based on continuous wavelength variations and the overlapping sensitivities of our cone receptors. The colors we identify, such as those in a rainbow, are part of a seamless gradient rather than isolated bands, which is why there is no separate color perceived between indigo and violet.