The beads, these beads give it a unique property

The ability to become invisible and not been seen has always been popular but especially in pop-culture with the rise in science-fiction and fantasy. There has been a lot of research into different areas in physics such as ray optics toachieve this effect in real life. There are a few approaches that can be taken to achieving invisibility such as bending or reflecting the light around the object you want to hide or recording an image of the surroundings and projecting it on the disguised object. Apart from the obvious task, challenges occur when developing the technology such as keeping it invisible for all angles it is viewed from, cloaking in a dynamic system and hiding large objects. Currently there are three promising approaches to accomplishing invisibility.Optical camouflage is the most popular process of cloaking and the easiest to achieve. The general idea is to record the background behind an object or person, process the image through a computer and then project the image onto said object or person, specialized equipment is required. A cloak made from retro-reflective material covers the object that is being disguised, the material is completely covered in tiny beads, these beads give it a unique property such that all light that is incident is retro-reflected. ‘The glass beads act like prisms, bending the light through refraction so that the reflected light travels along the same path as the incident light ray’s’ 1, this means that when at the light source an observer sees a brighter reflection. This gives the image the ability to be seen in bright conditions and from far distances. While the computer is not specifically specialised, software is needed to calculate the perspective required to simulate the background and convincingly display the image. To complete the image a beam splitter is needed to both reflect and transmit the light, this allows someone to observe the reflected image and the light from the surroundings making the two images fully entangled giving the most realistic view. While not the most practical method for creating an invisibility cloak there are many real-life applications for the technology such as in planes to give pilots a better view of runways for take off and landing, or even used in cars for parking/manoeuvring. Limited as the technology might be such as it only being able to do a 2-D image and in dynamic systems the projection will only be as fast as the computer able to render the new images, it is inexpensive compared to the other options and works in allowing a spectator to see through something.Recent experiments in ray optics have lead to designs able to hide centre meter to metre-sized objects. Using certain arrays of standard lenses, scientists from the University of Rochester were able to cloak objects in a specific region when viewed through these lenses 2. The light gets refracted when passing through the lenses so that the rays path when exiting the incident lens leads it to the opposite beside of the next lens (if the light enters at the rightmost side of the first lens, after being refracted that specific ray’s path will be directed at the leftmost side of the second lens), the final lens refracts the light, so it comes out of the lens on its initial path before passing through any of the lenses. Due to the light passing through many lenses (4 in the given example), there is a large region of space where the light does not pass through and so any object in this region will not come into contact and reflect the light therefore making said object invisible.   Fig.1 Displays the paths the rays take through all the lenses as well as the region able to hide objects 2The design uses no new materials, just plain lenses. However, for this to work the lens can only be viewed from a small range of angles, the lenses also must be placed at certain distances depending on the radii of curvature for each lens 2. Unfortunately, it is impossible to achieve this effect at all angles and thus not likely be a viable option for an invisibility cloak but designs such as these may be used for situations where precision is needed for instance in operations when surgeons want to see through their hands.There has been a relatively new study in physics that allows a great deal of development in physically bending light around an object, this is known as ‘metamaterials’. These metamaterials are designed and fabricated as they cannot be found naturally therefore giving them properties that cannot be found naturally. In the region of creating an invisibility cloak a material need’s to be created with special Electromagnetic properties, specifically to control the propagation of EM waves. Success has been achieved with metamaterials to effectively hide a 2-D cylinder from a small range of microwave radiation, this was achieved with a specific design similar to a split ring 3. The design allowed for negative magnetic response and the wires they were printed on gave a negative electrical response, and as theorised when achieved together the cylinder acted almost transparent to the electromagnetic waves and was said to have a ‘negative refractive index’ 3. Although there was some loss in intensity of the waves there was no immediate effects that displayed that an object was in its path. When tackling the problem for the visible spectrum of EM radiation the problem becomes a lot harder as you must try and develop the material or structure it in a specific way to make the object transparent for the whole of the visible spectrum, not just a specific range. Research into overlapping these designs to accompany the whole spectrum is ongoing yet not fruitful in their results, not to mention making the solution 3-D which is the harder problem. Other variations on metamaterial designs are in the works currently, one such design wishes to create a ‘mirage’ and hiding the object behind this fake image, this would be accomplished by grading the refractive index in a cloak to bend the lights path by just the right amount so that it misses the object while still leaving in the same path it entered, commonly referred to as a ‘carpet cloak’ 5. Results currently only work for the microscopic scale, however further research and development will likely lead to larger cloak’s being able hide objects on a larger scale.    Fig.2 Displays the wave patters for when the cloak is a) off and b) on 4Though currently the furthest away from achieving an invisibility cloak, metamaterials offers the largest opportunity in the future to create an invisibility cloak in the same way that is generally thought of in pop-culture as when the material is designed there is no need for preparation to hide objects, a flaw that does seem to be inevitable is that if a person was in the cloak they would not be able to view the outside of the cloak as no light reaches the centre of cloak.In conclusion, the reality of a practical invisibility cloak for human being sized objects is in the far future. While currently the most effective method is optical camouflage with its ability to cloak objects of any size (to an extent) given a large enough cloak, however this does not have the prospect of improving much more as the technology is currently very dependent on the quality of the computer’s hardware and software. Optical camouflage will likely be the most widely used method to see through objects due to it being relatively cheap to achieve. The fascinating designs using ray optics have lead to the ability completely hide objects up to a metre in size, although this is the least likely to be used in invisibility cloaks and has the smallest possible of improving in technology as there is no foreseeable future where a solution is discovered to overcome the many angles conundrum it can still be used in real life situations as described before. Finally, metamaterials have the most promising future for a solution, there is still ongoing research and I’m confident there will be research in the years to come into not only how these materials can be used to create an invisibility cloak but into a myriad of other technical problems. Still in its early stages with its current success onlybeing the with microwave radiation, on a single plane and inthe nanometre scale range. There is no lack of ability to improve in the area, however will take time. I do feel confident in saying that we will have the technology to achieve invisibility in the future.References1. V. Srikanth, P. Ramesh, Invisibility System Using Image Processing and Optical Camouflage Technology, IJETT, Vol 4, Issue5, Page 1874-18772. J. S. Choi, J. C. Howell, Paraxial Ray Optics Cloaking, Optics Express, Vol 22, Issue 24, Page 29465-294783. D. Schurig, J. J. Mock, B. J. Justice, S.A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith, Metamaterial Electromagnetic Cloak at Microwave Frequencies, Science, Vol 314, Issue 5801, Page 977-9804. M. Gharghi, C. Gladden, T. Zentgraf, Y. Liu, X. Yin, J. Valentine, X. Zhang, A Carpet Cloak Device for Visible light, 

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