A CONFERENCE commemorating John Tyndall was held in Dublin last month.
This great Irish scientist was the first to show that certain gases act like a blanket, trapping heat under them, a key process behind global warming. He also, famously, explained why the sky is blue and why a drake mallard’s head has such extraordinary colours.
Tyndall was born of modest land-owning stock in Leighlin-bridge, Co Carlow, where his father ensured he had a good education. The young Tyndall worked as a draughtsman and surveyor with the Ordnance Survey in Dublin and London before going to Germany to study. His professor at the University of Marburg was Robert Bunsen, whose ‘burner’ is still used in school science labs today. Tyndall went on to become a professor at London’s Royal Institution where he spent most of his career. A man of many parts, he was a keen mountaineer, being the first person to climb the 4,506m Weisshorn in the Swiss Alps. He might also have conquered the notorious Matterhorn, but local guides were too fearful to assist him.
Tyndall was fascinated by colour, something which doesn’t actually exist. We have little cone-like structures in our eyes which respond to light of particular wavelengths. The brain converts these inputs into the sensations we call colours, giving us a virtual reality model of the world. The light from the sun is a mixture of all wavelengths. When it passes through a liquid or a gas, the rays may collide with particles it contains. A ray with a length close to the thickness of a particle can be deflected. Penetrating ability is proportional to the fourth power of the wavelength, which means that short waves are interrupted and scattered much more than long ones. Radio waves, which have much longer wavelengths, behave similarly. RTÉ’s Tullamore transmitter, for example, sends out very long waves which can pass through most solid objects, whereas the much shorter wavelengths of television or mobile phone transmissions are easily blocked. AM radio, therefore, is available just about anywhere, but mobile networks, despite their many relay stations, have endless coverage problems.
In 1859, Tyndall shone a light through a vessel of water with little particles suspended in it. The blue component of the light, with its shorter wavelength, was deflected and scattered much more than the other longer-waved ones. Likewise, the blue component of light entering the atmosphere from space is scattered and deflected in all directions and so a cloudless sky looks uniformly blue. Other, more mundane, examples of this ‘Tyndall Effect’ include the smoke from old clapped-out car engines. It’s blue because exhaust contains tiny particles which drift in the air causing scattering. Eyes are blue for a similar reason.
When a drake mallard turns its head and neck, the feathers change colour as if by magic. Bright blues give way to a purple sheen. Another movement and the feathers seem to be red. The effect depends on the angle at which the light strikes the feathers. Like the water in Tyndall’s tank, they contain little particles. In a feather, however, these are not distributed randomly but arranged in layers from whose surfaces the light is reflected.
All large creatures, except burrowing moles and deep-sea fish which live in complete darkness, manufacture colour pigments. Nature’s make-up is usually spread along the surfaces of skin hair or feathers like a coating of paint. Water birds, such as ducks and kingfishers, adopt the much more elaborate Tyndall scattering approach. But why do they go to such trouble? It’s tempting to think they do so because their feathers are so frequently immersed in water they are prone to fading. But this can’t be the sole explanation. Birds as diverse as pigeons and parrots, which don’t often get wet, do the same. Tiny hummingbirds are Tyndall enthusiasts. Iridescent colours, like the flamboyant tails of peacocks or the antlers of red deer stags, seem to impress prospective partners in the cut-throat mating game. Are they bird equivalents of the latest creations of the Paris or Milan fashion houses?
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, November 28, 2011
