In his 1897 novella, "The Invisible Man" H. G. Wells speculated that it might be possible for a man to become invisible by manipulating his refractive index. Little over a hundred years later science is close to making his dream a reality, and as Wells predicted, controlling refractive index is the key.
The refractive index is a measure of by how much light is slowed down when it passes through a medium such as air, water or glass.
"Hold on," you might be saying, "Didn’t Einstein say that the speed of light is a constant?"
Well yes it is, in a vacuum, but light moves slower in a denser medium such as water. This gives rise to an interesting effect when light passes from one medium to another with a different density: it changes direction.
This is what makes a pencil seem to bend when it’s dipped into a beaker of water. As light from the pencil passes from water to air it shifts direction. This shift happens in a very predictable way that results from the properties of the material and the wavelength of the light. In fact different wavelengths bend by slightly different amounts, which is how a prism can split white light into a rainbow of colors.
Back in 1967 a scientist by the name of Victor Veselago theorized that it might be possible to manipulate the refractive index of a material to give it unusual or strange properties. Such materials don’t exist in nature but Veselago proposed that it might be possible to engineer structures to refract light and other electromagnetic wavelengths in very specific ways. It was this pioneering work that gave rise to what are now called metamaterials.
Since 1999 scientists at Duke University in the USA and Cambridge in the UK have been constructing metamaterials that have a negative refractive index. So far these have only been designed to operate on longer wavelengths that are invisible to our eyes, like microwaves, but work is progressing on structures that can manipulate visible light.
Scientists believe that these new materials will give them the power to bend and control light in ways that H. G. Wells could only dream of. In particular, it is thought that metamaterials could be constructed that would guide light over and around their surface rather than letting it reflect away as happens naturally. The analogy often used to describe this is of a rock in a river: water flows around the sides and joins back together. Downstream of the rock there is no hole in the water.
When a material is built that can do this, it will be invisible. It hasn’t happened yet, but the day is coming closer.
So does this mean the invisibility cloak will be a reality?
Well not quite. A cloak suggests a kind of fabric that flows to hide the wearer, but at the moment metamaterials are rigid structures. It is conceivable though that one day metamaterials might be used to hide something like a tank or ship. Invisibility is coming, so look out!
And to learn more about H. G. Wells: http://www.online-literature.com/wellshg/