Most people cannot distinguish indigo from blue or violet in an ordinary rainbow.

The longer version

Indigo occupies a narrow slice between blue and violet, and the cone cells that report those hues overlap heavily in that range. As a result the signal change across the boundary is small compared with the changes elsewhere in the bow, so the eye simply folds it into its neighbours.

Ask people to point out indigo in an actual rainbow and answers scatter widely. Color naming experiments find that English speakers agree closely on focal red or green but far less on boundary terms. Indigo survived in the school list mostly through tradition rather than through anything most people can pick out.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.

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