Invisibility cloaks


The invisible woman by marvel comics, Art by Steve McNiven Image from wikipedia

Sometimes its better to be unvisible.
This is not so impossible.
I.e. the theory is developped at various places, e.g. here at St. Andrews by physicist Ulf Leonhardt who has an easy to read introduction to the field.

If you put a straw into water it looks as if it is buckled – this phenomen is called refraction (see an image in the link). The buckling can be explained by the fact that light travels at different speed in materials with different refractive indices. Means: the straw is still straight but it appears buckled since the light which goes from the straw to your eye is bend since air and water have different refractive indices. However if the materials have GRADUALLY different indices then light may be bend in a curve. Now imagine that you have a material where you can “bend away” all light. Then you have an invisibility cloak.

In particular the effect is different from optical camouflage, where the background is projected onto the “invisible” object.

The first toy-application of this is introduced in this science mag article where a copper cylinder is ‘hidden’ inside a cloak. (see also D.R. Smiths homepage)
The herein used cloak is constructed using artificially structured metamaterials, designed for operation over a band of microwave frequencies. (microwaves are just as light electromagnetic waves, just with a longer wavelength). The design of the material is such that it allows for unusual refraction properties. In particular the refraction index of the material is varied in such a way that the microwaves are bend away from the cylinder.

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