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
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
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.
Click here to return to the Physics Archives
Update: June 2012