Optical Invisibility Cloak Hides Object's Shadow In Diffusive Media
13 June 2014
By using diffusive media, researchers from the Institute of Applied Physics at Karlsruhe Institute of Technology (KIT) in Germany have been able to develop an optical invisibility cloaking method that can conceal objects' shadows.
The experiment conducted by the scientists included a back-lit Plexiglas tank that was just a few centimeters wide. It was filled with a white, cloudy liquid to provide diffusion. Within that liquid, light is scattered several times and because of that, its effective speed is reduced.
Metal cylinders were placed within that liquid and at first they produced shadows on the tank walls. To make light pass around the cylinders, researchers coated them with a thin shell of transparent polydimethylsiloxane (PDMS), a type of silicon material, which was mixed with light-scattering melamine microparticles.
This shell sped up diffusion around the coated object compared to its water environment so light went around the cylinders and they no longer cast a shadow. The team also made a cloak that concealed a spherical object.
One possible application of the optical cloak would be to produce frosted bathroom glass panes containing visually concealed metal bars or sensors against burglary. Still, according to researchers, it would take much perfecting of the cloaking technology, which at this point has only been proven in principle by the experiment.
KIT professor Martin Wegener pointed out that the existence of ideal invisibility cloaks in the air - and not in cloudy water like in the experiment - would violate Albert Einstein's theory of relativity that prescribes an upper limit for the speed of light.