Seeing colour
Structural colour
Colour made by tiny structures, not pigment. It is why a peacock feather shimmers and why no bird has blue pigment.
Presented by ClearTrust Free for designers, students and the curious.
Most colour comes from pigments, which absorb some wavelengths of light and reflect others. Structural colour is made a different way. Surfaces with ridges, layers or sponge-like patterns thousands of times thinner than a hair bend and reinforce certain wavelengths. The result is often brilliant and changes with the viewing angle. Peacock feathers, morpho butterflies, beetle shells and soap bubbles all work like this.
How structure makes colour
When light meets a very thin transparent layer, some reflects from the top surface and some from the bottom. The two reflections overlap. Where their waves line up, that colour is strengthened. Where they are out of step, it is cancelled. This is called interference. A soap bubble and a film of oil on a wet road show it with one layer. Nature stacks many layers, or builds regular lattices, to get much purer colours. Robert Hooke and Isaac Newton both described the effect in peacock feathers in the seventeenth century.
Blue without blue pigment
Blue pigment is rare in animals. The blue of a kingfisher, a blue jay or an Indian roller comes from a spongy arrangement of protein and air inside the feather that scatters blue light, with a dark layer of melanin behind it. Grind such a feather to powder and the blue disappears, leaving dull brown. Blue human eyes work in a similar way: they contain no blue pigment, only scattering in the iris. The greens of many parrots and frogs are a structural blue seen through a layer of yellow pigment.
Why people copy it
Colour that changes with angle is called iridescence. It is seen in pearls, abalone shell, jewel beetles and the throats of hummingbirds and sunbirds. Because no pigment is involved, structural colour does not fade. Beetle wing cases used in embroidery in India and Thailand centuries ago are still bright, and fossil beetles millions of years old keep traces of their sheen. Engineers copy the principle for paints without toxic pigments, for security marks on banknotes and for anti-reflective coatings, which use interference to cancel reflections instead of creating them.
Sources and further reading
- Structural coloration on Wikipedia
- Scientific American: Understanding How Animals Create Dazzling Colors
- Zi and others, Coloration strategies in peacock feathers, PNAS (2003)
- Vignolini and others, Pointillist structural color in Pollia fruit, PNAS (2012)
- Webexhibits, Causes of Color: Peacocks
Original text by the Rang desk, written with AI assistance and checked against the sources above. Meanings of colour vary from place to place and person to person. If we have something wrong, tell us.