Invisibility Cloaking ```Name: Kate Status: student Age: N/A Location: N/A Country: N/A Date: N/A ``` Question: In regards to invisibility cloaking, I read that to cloak a material from light, the speed of light has to be infinitely fast at the inside of the cloak and therefore the refractive index must tend to zero. However, I read here http://news.bbc.co.uk/1/hi/sci/tech/8574923.stm, that an invisibility cloak was produced that worked by varying the refractive index based on the ratio of polymer to air to achieve a varying refractive index of between 1 to 1.52. As the refractive index in this material does not go below 1 does that mean that there is an alternative way of cloaking where the speed does not have to tend to infinity? Or does this method have limitations in efficiency i.e. higher loss, more shadowing or smaller bandwidth of frequencies that can be cloaked? Replies: Kate, The notion that the speed of light is "infinite" is completely disregarded by Einstein's equation E=m*c^2. In other words, Infinite speed where c is the speed of light, would mean infinite energy, which is theorized today to be a Black Hole. When considering the changing of the refractive index of certain materials, you are nowconsidering the geometries of certain materials that "bend" light based upon the material's crystal structure. There are experiments that have taken gold and reduced it to particles in the micro and nano scales. Meaning the gold's molecular or crystal structure are in the micro-meter or nano-meter scales. Gold in it's natural state, bend or reflect light, and make gold appear "gold". However, break up the gold particles to smaller scales, in the micro or nano realm, where the wavelengths or frequencies of light are bent or reflected in a certain way, and you'll discover different colors, or different wavelengths or frequencies of light, that display "red" or "blue", but not "gold" Because of this result, there maybe some kind of material, whose molecular structure can be reduced to the scales in the nano world, that bend light with a refractive index of 1, and hence your cloaking material. Hope that helps. -Alex Viray Click here to return to the Physics Archives

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