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Invisibility Cloak For Real This Time?

        Science | Optics

May 25, 2006

Image courtesy © NewsCom
If Clay had an invisibility cloak, he wouldn't have to be a fly on your wall -- wait, he already is.

Here it is, folks -- another plan to make the dreams of Trekkies, Harry Potter fans and Clay Aiken come true. And no, it doesn't have anything to do with secret mental techniques to disappear from a staff meeting.

Sir John Pendry of London's Imperial College and a trans-Atlantic team of scientists are working in tandem to develop a true invisibility cloak. When completed (projected to happen around 2011), the cloak will work like it does in the movies: once covered, any object or person appears to vanish completely.

We reported on a different invisibility cloak in 2005 that uses special retro-reflective fabric, a video camera, a computer and a projector to render its wearer (almost) invisible. The retro-reflective fabric is actually opaque and comprises many tiny beads. While other materials reflect or diffuse light, this cloak sends it right back to the source. But all that does is create a brighter reflection. That's where the computer, video camera and projector come in. The invisibility cloak becomes the projector's screen and merely shows what the camera, stationed behind the wearer, captures.

Image courtesy ©Tachi Laboratory, the University of Tokyo

To better understand what's happening, here's a summary of surface reflectivity from How Light Works:

When a light wave hits an object, its outcome depends on the energy of the light wave, the natural frequency at which electrons vibrate in the material and the strength with which the atoms in the material hold on to their electrons. Based on these three factors, four different things can happen when light hits an object:
  • The waves can be reflected or scattered off the object.
  • The waves can be absorbed by the object.
  • The waves can be refracted through the object.
  • The waves can pass through the object with no effect.
More than one of the above possibilities can happen at once.

In absorption, the frequency of the incoming light wave is at or near the vibration frequency of the electrons in the material. The electrons take in the energy of the light wave and start to vibrate. What happens next depends upon how tightly the atoms hold on to their electrons. Absorption occurs when the electrons are held tightly, and they pass the vibrations along to the nuclei of the atoms. This makes the atoms speed up, collide with other atoms in the material, and then give up as heat the energy they acquired from the vibrations.

The absorption of light makes an object dark or opaque to the frequency of the incoming wave. Wood is opaque to visible light. Some materials are opaque to certain frequencies of light and transparent to others. Glass is opaque to ultraviolet light, but transparent to visible light.

Pendry and his team's cloak won't need the video equipment of its predecessor. Their cloak, really an invisibility shield because of weight and thickness, will be made of metamaterials that don't absorb or refract light, but redirect it. Currently, the team is only capable of working with wavelengths larger than light. They have the statistics to prove that invisibility is possible, but at the moment, it's still theoretical. In the meantime, there's always invisible ink -- oh, and this guy.

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