The iridescent colours found on a butterfly's wing may soon be appearing in the clothes we wear.

British scientists are studying the optical trickery butterflies use to produce their spectacular colours.

They believe the results may lead to spray-on iridescence for the fashion industry. Other applications could be new liquid crystal displays and anti-counterfeiting marks for banknotes.

Butterfly colours are not all pigments

Butterflies and moths produce some of the most brilliant colours in nature. The colours are often not due to pigments but are created by tiny structures that manipulate the very nature of light itself.

The trick is called iridescence and the secret lies in the detailed structure of the wing.

"Butterflies and moths have got iridescence down to a fine art. We want to explain the structures found within the scales on their wings that cause the colours," explains Professor Roy Sambles of Exeter University.

"We hope eventually that our research will allow us to design novel synthetic structures that have the same properties," he adds.

Remarkable light show

Butterfly wings consist of intricate layers of chitin cuticle separated by air spaces. These can be seen using the magnifying power of an electron microscope.

"A solid block of chitin is almost transparent, but when it is arranged in specific geometrical layers, it can reflect light in a remarkable way," said Dr Pete Vukusic of Exeter University, who has been studying the scales.

"The diversity and complexity of these chitin arrangements place them among the most complicated extra-cellular structures anywhere in the living world," he added.

Two excellent examples of iridescence of butterflies and moths are the wing scales of most of the Morpho family of butterflies and the Urania family of moths. These were among the first to be examined because of their brilliant iridescence.

Layers in Christmas tree patterns interfere with the light

Each Morpho scale has a series of cuticle ridges arranged in a "Christmas tree" formation. Each layer reflects light in the same way as a thin film of oil on water. The effect of several layers produces multiple light reflections.

The well-known effect of light interference causes some colours to be cancelled out while others are reinforced. The result is the striking colours we see.

The colours depend upon the exact structure of the wing and also the angle at which it is observed. Morpho can be bright blue when seen from above but looking from an angle makes it appear more violet.

As well as using an electron microscope the Exeter research team have also examined butterflies wings using lasers. They have found that some wings can reflect as much as 75% of blue light, a remarkable value for a natural substance.

Sparks of colour

Iridescent colours have long fascinated scientists. In 1634 Sir Theodore de Mayerne, physician to Charles 1, observed that the "eyes" on the wing of the peacock butterfly "shine curiously like stars, and do cast about them sparks of the colours of the Rainbow".

Sir Isaac Newton in his book Opticks, published in 1704, put forward a reason for the iridescent colour from the feathers of peacock tails.