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Interesting Findings And World Unfolding Through My Eyes.

Monday, April 23, 2007

Magic And Mystery Of Not Being Seen

Humans have long been fascinated by the concept of invisibility. From H.G. Wells' Invisible Man to Harry Potter's cloak of invisibility, purveyors of fiction have pondered what one would do if one could move about unseen. Invisibility is often portrayed as a perfect transparency– ala the Invisible Man– however this method is in conflict with the laws of nature as we understand them. Moreover, a transparent person would be plagued with a host of difficulties that seem quite insurmountable. Any consumed food or drink would be embarrassingly visible as it meanders through the digestive system, and these visible nutrients would immediately begin to integrate into the body. That's to say nothing of wardrobe problems and social difficulties.

The competing approach to invisibility involves some sort of cloaking device to route photons around an object. This method is somewhat more feasible, but of course it comes with its own unique set of complications. For instance, if all the outside light is diverted around something, no light is able to reach an observer inside, leaving them unable to see out.

These difficulties and others have long left all serious speculation about invisibility lodged safely in the distant future. But this is no longer so. In October of 2006, Professor Sir John Pendry of the Imperial College London announced the successful creation of a rudimentary cloaking device which nudges the idea a bit closer to reality. Perhaps most surprising of all, the whole concept rests on a fairly simple physical principle of light– one that requires no electricity to operate, and that every high-schooler learns in basic physics.

In essence, Sir John's invisibility cloak relies on refraction, the same property of light seen when a prism casts a rainbow. Refraction can also be seen by poking a pencil into a glass of water. The underwater portion will appear to be offset from the rest because of the bending of light as it moves from one medium to another– from water to air. A few years ago Sir John and his physicist friends reflected upon the idea of using refraction to bend light completely around an object. If this were possible, the light would emerge on the other side, unchanged, as if the object were not there at all.

Refraction through waterOf course this simple idea isn't quite so simple in application. The researchers' first obstacle was the precision light-bending this method requires. There simply weren't any materials with quite the right properties to bend light in the necessary semi-circle, nor were any naturally occurring materials good candidates for the position. So the scientists looked to metamaterials– substances whose electromagnetic properties are dependent upon tightly designed internal structures rather than on their chemical composition.

Guided by a theoretical design published in an earlier paper, and working in concert with researchers at Duke University, Sir John and his team created a five-inch round cloak using a metamaterial structured in two-dimensional concentric rings, specifically designed for this purpose. This unique configuration is thought to be one of the most complex metamaterial structures ever made. Their first goal was to make a material that was "invisible" to microwave radiation since microwaves are a longer wavelength than visible light– millimeters rather than nanometers– and therefore easier to manipulate.
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