“I am Harry Potter and this is my invisibility cloak! Look – now you see me… and now you don’t!” The eleven year old me gushed as I was getting ready for a night of trick-or-treating.
“It is just a black cloak wrapped around you, I still see you.” That was when I realized that invisibility was only possible in Harry’s magical world.
Or is it?
Recently, I stumbled across an article that claimed that scientists have created an invisibility cloak by using the ‘mirage effect’. Propelled by my childhood dream to possess such a cloak, I began to delve further into this topic. Before we can move into the science behind how such a cloak is made, we need to first understand the concept of refraction, how the human eye perceives objects and how this leads to the mirage effect. Humans are able to see an object because light waves are reflected or refracted (bent) from the object and then travel to the human eye. Sometimes, however, light waves from an object pass through another medium and bend the light wave another direction. Imagine, for instance, a spoon inside a glass of water – when inside a glass of water, the spoon appears to be ‘broken’. This is because the light waves reflected from the part of the spoon submerged under water, are refracted when they pass through the surface of water. Unfortunately, our brain does not know the light waves from the spoon have been refracted, and thus we perceive the spoon to be at different position under water. Figure 1 below further explains this concept.
Figure 1: Concept of Refraction
The mirage effect is based off this concept. Many of you must have probably experienced driving down a road on a hot summer day and seeing a pool of water in the distance, only to realize that it was actually a mirage. Mirages form because of a temperature gradient between the air and surface of the ground. Usually, light waves from the blue sky are reflected off the surface of a road and thus allow us to see the road ahead. However, in a mirage, a very hot surface causes the light waves from the sky to refract before coming in contact with the road. Since our brain does not know the light wave has been bent, the eye traces the light wave in a straight line to the ground, thus causing our eyes to incorrectly perceive the light waves as a pool of water in the distance (when it is actually refracted light waves from the sky).
Using the concept of the mirage effect, scientists have made an invisibility cloak out of a lattice of carbon nanotubes that when electrically stimulated, either by electrical heating or by a pulse of electromagnetic radiation, create a temperature gradient that cause light waves to bend away from whatever object is under the invisibility cloak. The most important aspect of such an invisibility cloak is the lattice of carbon nanotubes. In order to bend visible light waves, the lattice of carbon nanotubes (also known as metamaterial) must be spaced less than the wavelength of visible light. Till now, researchers have only been able to succeed with near-infrared radiation as our technology is not sophisticated enough as yet to create a lattice with smaller spaces between the carbon nanotubes. Thus, until scientists are able to create a lattice small enough to bend light waves from the visible spectrum, an object will remain visible to the human eye.
Figure 2: An object covered in an invisibility cloak made of carbon nanotubes that bend the light waves around the object, making the object invisible.
So what does all this really mean? Could Harry Potter’s invisibility cloak really exist? In the future, perhaps yes. Yet, there are even bigger implications of a possible invisibility cloak – good and bad. Using metamaterials to bend light waves, society could improve its security by placing ‘invisible’ policemen around each city. A country’s military could also benefit from such technology as tanks and airbases could be hidden from the human eye. However, such an invisibility cloak could also increase crime rate in the future as this technology could be further developed to bend sound and magnetic waves as well, allowing terrorists carrying guns or bombs to walk through metal detectors undetected. This could arouse an ethical debate over the use of metamaterials and invisibility cloaks. Yet, the debate can wait till the day researchers create the first cloak invisible to the human eye.
1. “HowStuffWorks “Metamaterials: Bending Light Waves”” HowStuffWorks “Science”Web. 07 Oct. 2011. <http://science.howstuffworks.com/invisibility-cloak6.htm>.
2. “Researchers Create Functional Invisibility Cloak Using ‘Mirage Effect’ | Fox News.” Fox News – Breaking News Updates | Latest News Headlines | Photos & News Videos. Web. 07 Oct. 2011. <http://www.foxnews.com/scitech/2011/10/05/researchers-create-functional-invisibility-cloak-using-mirage-effect/>.
3. “How Do ‘invisibility Cloaks’ Work?| Explore | Physics.org.” Physics.org | Home. Web. 07 Oct. 2011. <http://www.physics.org/article-questions.asp?id=69>.