The Colour in a Peacock's Feathers

Nov 18 2013

When I was a child my two granduncles, Uncle James and Uncle Charlie, lived in a house with the most incredible garden. Uncle Charlie was, and still is, an excellent gardener and there was an abundance of beautiful, brightly coloured flowers. Uncle James was a keen ornithologist and he kept a number of tropical birds, including a peacock. We visited them only occasionally and entering the garden, populated by vibrant flora and incredible, exotic creatures, felt like entering a magical wonderland. But the thing I found most fascinating on our visits was the peacock. When the bird displayed his tail feathers the array of eyes blinking at me seemed supernatural, the colours of the peacock’s feathers were so intense and striking, stronger and truer somehow than most of the other colours around me. It was years later that I gained some understanding of what it is that makes the colours on a peacock’s tail feathers so special.

We observe variations in colour because our eyes can distinguish between different wavelengths of visible electromagnetic radiation (the visible range of radiation makes up only a small part of the electromagnetic spectrum). We observe longer wavelengths as reds and shorter ones as blues and violets. Our main light source, the sun, gives out radiation ranging from ultraviolet to infrared, which includes the visible spectrum, and much of this reaches us here on earth and allows us to see as well as to not freeze to death (infrared) and to get a suntan (ultraviolet).

Usually when we see something as a particular colour it is because it absorbs certain wavelengths of light and reflect others. This is sometimes referred to “ordinary colour” and accounts for most of the colours that we see, for example, in paints, in fabrics and in the flowers in Uncle Charlie’s garden.

But sometimes the reason we see different colours is much more interesting. The striking colours of the peacock’s feathers, that appear to change as the feathers move, are due only in part to “ordinary colour”. The feathers possess a nanostructure – a repeated structure so small and precise that, while it is invisible to the naked eye, it interacts with light waves to change the way we observe them. This is called “structural colour” and is what is responsible for the iridescent quality of a peacock’s feathers. The nanostructure of the feathers means that when light hits them it is reflected off them at different points, so a number of different light waves is reflected back. The reflected light waves interact with each other and result in interference. This means that if the waves are in phase, in other words if they have their peaks and troughs at the same points, they will be amplified; if they are not in phase they will be partly or entirely cancelled out. Because light waves with different wavelengths appear as different colours, some colours are amplified and some are cancelled out, depending on the feather’s structure. As you, the observer, move the colours appear to change as different wavelengths are amplified in the direction of the viewer.

Peacocks are not alone in nature in possessing structural colour. The striking beauty of the butterfly’s wing, the delicate iridescence of mother-of-pearl and the magnificent shells of some beetles are other examples. Scientists have learnt a great deal from studying these natural examples of structural colour and many attempts have been made, with some success, to recreate these colours artificially. So far, however, I have never observed a man-made colour that has possessed quite the same magical quality as Uncle James’s peacock.

Dr. Eva McGuire is a postdoctoral researcher in the School of Physics in Trinity College Dublin.

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