The wings of iridescent butterflies are shaped like Christmas trees and this gives the creatures their colours, according to researchers at the University of Exeter.
Butterfly wings are covered in a myriad of scales. The colours of British butterflies come from plain dyes but iridescent tropical butterflies use the scales on their wings to create colours by reflecting and scattering light. Roy Sambles and his team at the University of Exeter have used electron microscopy to study the way in which these scales are arranged.
The researchers have found that in some species these scales - which are just a tenth of a millimetre across - are arranged in layers like a pack of cards, separated by thin layers of air. In others, they are arranged like the branches of Christmas trees.
The scales form long ridges on the wings. Professor Sambles and his team cut through these ridges to expose their structure. They found that the scales closest to the wing were larger than those furthest away and that the whole lot were held in place by a supporting "trunk".
"The primary cause of iridescence is interference - like the pattern you get from oil on a damp road," said Professor Sambles. Each layer of scales reflects the light, generating multiple reflections. The light waves interfere with each other, cancelling out some colours and reinforcing others. In the case of the electric blue butterflies studied by Professor Sambles and his colleagues constructive interference boosts only the blue light.
Other butterflies combine light of different wavelengths to produce a new colour. "There is a silver butterfly that combines red, green and blue light to create white light that appears silver, as the butterfly is vividly iridescent," said Professor Sambles.
The iridescence also depends on the angle at which you look at the wings. The electric blue butterflies appear violet when viewed from the side, as the light is shifted towards the ultraviolet. "We want to know if we can make bright materials using this method," said Professor Sambles. "Imagine a swimsuit or dress - that would be nice." Other possibilities include paints that are not dye-based, and wide-angle liquid crystal displays.
The work is partly funded by the Defence Evaluation and Research Agency, which is interested in using the techniques for camouflage. Surfaces coated with scales could also confuse - or correct - radar signals.