INVISIBILITY- A POSSIBILITY?
Have you ever read books like “Harry Potter” and “The Invisible Man” and found yourself wishing that you could turn yourself invisible? For many years, scientists have been working to turn that dream into a reality, and it looks like they’ve finally come one step closer to achieving it.
Before we get into the science of invisibility, let us begin by understanding how our vision works. Normally, we see objects because light hits an object, bounces back, hits our eyes and sends the image to the brain which then makes sense of the image and voila! you have a cup in front of you! The colour of the said cup depends on the colour of light that it reflects back — if it’s a green cup, it will absorb all wavelengths of light except green, which it will reflect and you end up seeing a green cup. Ok, as that’s out of the way, let’s get to the real deal.
Scientists have experimented with many methods over the years to achieve invisibility. One such method that was devised made use of a new man-made material called metamaterial. Metamaterials are made of repeating patterns of composite materials like metals and plastics at a scale smaller than the wavelength of the phenomena that they influence. The reason they are so special is that their geometry, orientation and arrangement gives them new properties that cannot be achieved in natural materials. One of the properties is manipulation of electromagnetic waves — whether it be blocking, absorbing, enhancing, or bending these waves. In this list of properties, the one which is most useful to achieve invisibility and the very reason for which these materials are used, is the ability to bend electromagnetic waves like light. So, now the question is, “Why is bending of light so important to this discussion?” Well, the answer to that can be explained by going back to the basics.
In order to see an object, light has to interact with it. Therefore, we can make the simple inference that in order to make an object invisible, light shouldn’t be allowed to interact with the object, but instead be made to bend around the object so that what you see is not the object, but what’s behind the object, thus making it seem invisible. This is where the concept of negative refractive index comes into place.
We have all seen and heard of “refraction”. It is essentially the bending of light when it travels between media of different densities. Normally, the refractive index of air is 1 and in other materials it is greater than 1. This means that when light travels from air to another material, it usually bends towards the normal drawn to the surface of the material.
Now, what if there was a way to manipulate the refractive index so that it becomes negative? This can be achieved by altering the internal structure of a material on a small scale, like in negative index metamaterials or left handed metamaterials to the point where we can switch the positive refractive index to negative, thus enabling us to manipulate the direction of bending of light.
In 2007, a research team at Maryland’s A. James Clark School of Engineering used plasmon technology to create the world’s first invisibility cloak for visible light. The invisibility cloak device was a two-dimensional pattern of concentric rings created in a thin, transparent acrylic plastic layer on a gold film and only 10 micrometers in diameter. But the cloak used a limited range of the visible spectrum, in two dimensions. It would be much harder to achieve invisibility in three dimensions since scientists would need to control light waves both magnetically and electronically to maneuver them around the hidden object. More recently, in the University of Texas, scientists have been successful in concealing an object to microwaves. It was called metascreen and was made of a 66 micrometre thick polycarbonate film that supported an arrangement of 20 micrometer thick copper strips which looked similar to a fishing net. In the experiment that was carried out , when the metascreen was hit by 3.6 GHz microwaves, it re-radiated microwaves of the same frequency that were out of phase, cancelling out reflections from the object being hidden. This could have a lot of applications in radio communications, weather tracking and in health care. In 2019, Hyperstealth Biotechnology had patented the technology behind a material that bends light to make people and objects near invisible to the naked eye. The material, called Quantum Stealth, is currently still in the prototyping stage, but was developed by the company’s CEO Guy Cramer primarily for military purposes, to conceal agents and equipment such as tanks and jets in the field. Unlike traditional camouflage materials, which are limited to specific conditions such as forests or deserts, according to Cramer this “invisibility cloak” works in any environment or season, at any time of the day.
Check out this video on Quantum Stealth:
Other than these technologies, methods like spectral cloaking and active camouflage have also been developed. In spectral cloaking, light isn’t made to bend around the object, instead it shifts one spectrum of light to another and then shifts it back once it reaches the other side. If for example, we take the cup we mentioned before and shift the green light to another part of the spectrum that isn’t reflected by the object, say blue, then no light will be reflected and the object will not be visible. The device then shifts the green light back to reverse the process once the light reaches the other side. This method eliminates any kind of distortion that could be caused by bending of light around the object and hence is more effective. The cloaking device is constructed from two pairs of two commercially available electro-optical components. The first component is a dispersive optical fiber, which forces the different colors of a broadband wave to travel at different speeds. The second is a temporal phase modulator, which modifies the optical frequency of light depending on when the wave passes through the device. One pair of these components was placed in front of the optical filter while the other pair was placed behind it. This method of spectral cloaking could be applicable to securing data transmitted over fiber optic lines and also could help improve technologies for sensing, telecommunications, and information processing .It could also be used for selectively removing and subsequently reinstating colors in the broadband waves that are used as telecommunication data signals which could allow more data to be transmitted over a given link, helping to resolve impairments due to increased demand in data.
In 2003, scientists at the University of Tokyo created a prototype active camouflage system that makes use of a material impregnated with retroreflective glass beads. The viewer stands in front of the cloth viewing the cloth through a transparent glass plate. A video camera behind the cloth captures the background behind the cloth. A video projector projects this image on to the glass plate which is angled so that it acts as a partial mirror reflecting a small portion of the projected light onto the cloth. The retroreflectors in the cloth reflect the image back towards the glass plate which being only weakly reflecting allows most of the retroreflected light to pass through to be seen by the viewer. This system works only when viewed from a certain angle.
In conclusion, even though an invisibility cloak that could make objects, much less humans invisible has not yet been perfected, we are on the right track. The very fact that we have come this far goes to show that with technology and a creative mindset, anything is possible.
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