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Hazing: How to hide in nearly plain sight

A new cloaking device can hide objects when a light fog is present

By
9:08am, June 24, 2014

Light moving around an object creates a shadow (the dark stripe at left). But when an invisibility cloak guides the light around the object (right), the shadow vanishes and the object becomes largely invisible. This technique works only in a haze — like fog or a cloud — where light bounces through a material like a pinball. 

R. SCHITTNY/KARLSRUHE INSTITUTE OF TECHNOLOGY

A new invisibility cloak can hide objects in semi-plain sight — sometimes. Unlike earlier cloaking devices, this one can conceal things from light of any color and coming from any direction. But that flexibility comes at a price: This cloak only works under hazy conditions, such as in fog, in a cloud or when viewed through frosted glass.

“It's a nice demonstration,” Jason Valentine told Science News. A mechanical engineer at Vanderbilt University in Nashville, Tenn., he did not work on the cloak. The new system “sacrifices functionality in one area to gain functionality in another,” he noted. He meant the cloak can work with all kinds of light but not in every situation.

A perfect invisibility cloak may forever be a figment of human imagination. “Employing a cloak in the Harry Potter–type fashion is not doable now and maybe not doable ever,” Valentine said.

Like many ideas, the basic plan behind most invisibility cloaks is easier to describe than to build. People can only see things that emit or reflect light. An invisibility cloak does neither of those things. The cloak guides light around a concealed object, like water moving around a rock in a stream. On the “downstream” side, the light resumes its journey, uninterrupted.

But there's a problem: Light takes more time to move around an object than to move directly past it. As a result, the redirected light can't catch up to the light that moves in a straight line. A person would be able to see that something had changed.

Robert Schittny at the Karlsruhe Institute of Technology in Germany came up with a device that avoids that snag. This physicist and his colleagues thought hiding an object in haze might help. That's because light can't zip straight through foggy or cloudy places. Instead, it moves more like a pinball. It bounces off tiny particles — like water — suspended in the air.

The German scientists hid objects inside a cloak that started with cylindrical and spherical shells made from stainless steel. They painted the shells white and covered them with a coating that contained tiny particles of a hard plastic called melamine (MEL-ah-meen) resin. The scientists then placed the shells in a tank of water mixed with particles of white paint. The paint particles acted as the fog.

When they shined white light through the tank, the shells cloaked themselves by guiding the light around their surfaces. From the other side of the tank, the shells were barely visible. Without the cloak, the shadowy outline of the hidden objects was easy to pick out. The researchers describe their cloaking system in the June 5 issue of Science.

This technology cloak may not be ready for prime time quite yet. Still, Schittny believes it might one day prove useful. One simple example: It might hide the wires used to reinforce some types of frosted glass.

Power Words

engineering    The field of research that uses math and science to solve practical problems.

melamine    A basic ingredient (made from ammonia and urea) in some resins. Manufacturers use those resins to make durable materials (such as inexpensive kitchen plates) and surfaces (such as those on some cars).

physics    The scientific study of the nature and properties of matter and energy.

plastic  Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

resin  A sticky, sometimes aromatic substance, often secreted by plants. It may also be the viscous starting ingredient for some plastics that will harden when heated or treated with light.

technology    The application of scientific knowledge for practical purposes, especially in industry.

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