Anyone making such a cloak would have to choose what form of radiation one wanted invisibility from. The invisibility would work both ways--a person hidden from the visible light spectrum would have to use infrared or sonar or microwaves to see out. Note: I believe they could leave a pinhole in the visible spectrum which would be difficult to spot that someone inside could use to look out.
More technical details at the MIT technology review The cloaking effect depends on a material's "refractive index," or its ability to influence the direction of light that passes through it. Light tends to prefer the quickest route between two points, which is normally a straight line. With metamaterials, however, the quickest path can be one that bends around an object.
But bending light is just one of the requirements for cloaking. "You have to return the light to the same path it was pursuing before it hit the cloak; otherwise it casts a shadow," says Pendry. Similarly, when light enters the cloak, it must not be reflected. "One way to think about it is that this material gives the appearance of being like space," says Smith, in that space can bend light and also has no reflection.
"It's a breakthrough," says George Eleftheriades, an expert in metamaterials at the University of Toronto. However, he says, there is a limitation: "It won't work for every frequency."
Indeed current materials are capable of redirecting only microwaves, which means the cloaking device Smith and Schurig are developing will work only against radar or other microwave emitters. While this is likely to prove useful for future stealth planes, we are still at least a decade away from cloaking objects from visible light.
It will be difficult to cover the whole visible spectrum. An object would be encased in a shell of metamaterials and they would create an illusion akin to a mirage, said David Schurig of Duke University in North Carolina, who worked on the second report. The light rays end up behind the object as if they had traveled in a straight line.
Cloaking could be used on space probes to protect sensitive equipment from cosmic radiation.