Modified invisibility cloak could make the ultimate illusion

Chalk Drawings and Optical Illusions

Don’t believe everything you see. Metamaterials could make a cup look like a spoon (Image: Edgar Mueller/Rex Features)

AN ILLUSION device that makes one object look like another could one day be used to camouflage military planes or create "holes" in solid walls.

The idea builds on the optical properties of so-called metamaterials, which can bend light in almost any direction. In 2006, researchers used this idea to create an "invisibility cloak" that bent microwaves around a central cavity, like water flowing around a stone. Any object in this cavity is effectively invisible.

Now a group of researchers has gone a step further. "Invisibility is just an illusion of free space, of air," says Che Ting Chan, a physicist at the Hong Kong University of Science and Technology and a co-author of the study. "We are extending that concept. We can make it look like not just air but anything we want."

Instead of bending light around a central cavity, the team has worked out the mathematical rules for bending light in other ways. For example, a material could be designed to bend light in the same way as a spoon would. So the light hitting the material would be distorted to make it look as if a spoon were there.

It is also possible to design a complementary material that has the opposite effect - to exactly cancel out the effect that an object has on light. So light distorted by a spoon could be passed through a complementary material to eliminate these distortions.

The new illusion device uses these two ideas together. To make a cup look like a spoon, for example, light first strikes the cup and is distorted. It then passes through a complementary metamaterial which cancels out the distortions to make the cup seem invisible. The light then moves into a region of the metamaterial that creates a distortion as if a spoon were present. The result is that an observer looking at the cup through the metamaterial would see a spoon (Physical Review Letters, vol 102, p 253902).