Abstract Invisibility has long been employed in works of science fiction and fantasy, from “cloaking devices” on spaceships in the various Star Trek series to Harry Potter’s magic cloak. But physicists and engineers are beginning to think they can actually make devices with the help of these materials. To achieve the feat of “cloaking” an object, they have developed two kind of materials that being used for invisibility, what are known as metamaterials, some of which can bend electromagnetic radiation, such as light and thermal and also optical lens, that able to bend the incoming light ray from in order to around an object, giving the appearance that it is not there at all. Introduction
Definitions and Background The desire to make objects or people invisible has a long history: to this end, many different devices have been imagined. However, it is only in relatively modern times that these plans have begun to move from fiction to reality. The recent development of metamaterials and the new design of optical lens have made accessible and opened up new possibilities for invisibility, the most promising of which are described in this article.
To be invisible in the conventional sense is to be undetectable to electromagnetic radiation within a given frequency range; light must not be scattered or absorbed. An immediate and unfortunate consequence of this requirement is that an invisible person must also necessarily be blind. In many applications, partial invisibility is sufficient. Hence, with all these kind of technology, metamaterials and optic lenses will provide the human being a wide range of new applications device and new systems whether in military sector or in automotive system such as, in Keio University, Tokyo for instance, enable the back seat driver to become invisible whenever the driver is moving backward (Gbur,2013) or when a jet fighter falls down in enemy terrain, the pilot could seek a hide with the help of Quantum Stealth (invisible cloak) made by the Canadian company (Cramer,2012).

Purpose and Audience The purpose of this paper is to study the materials used in invisible cloak and any related information regarding this topic. Throughout the paper, the materials used, the behaviour or the properties of the materials and also the limitations of these materials will be mentioned and discussed briefly. Moreover, this paper is intended for the technical experts like engineers and physicist. Moreover, this paper has been written with sub divided of technical properties so that the general public too can understand easily on how invisibility takes place.

Sources Some of the articles were obtained through the use of Western Michigan University’s online scientific journal database and few them from the online search engine like Google. All the collected journals were collected in ‘pdf’ files that will be submitted along with this paper in hard copy form. Almost majority of the sources came from engineeringvillage.com, sciencedirect.com and proquest.com.

Working Definitions There are several terms in the paper that the reader might not be familiar with and that are the important concepts that enhance the discussions and conclusions that follow.
These include:
1. Invisible Cloak – a type of camouflage garment where it can hide the object that is covered by itself, like Quantum Stealth, where it bends the light waves around the target (Cramer, 2012).
2. Metamaterials – Engineered materials made from nanoparticles to produce properties that able to control that intersect with them (Dana, 2014)
3. Optical Lens – Optical device that able the light rays to focus into one point.

Limitations This paper will discuss exclusively about the invisible cloak that became so common in today’s research. There will be no discussion about the technical aspect, such as the complex calculation of the wavelength, effect of the cloaking device and 3D simulation of the invisible cloak. In other words, this paper focuses on the material’s properties rather than technical aspect.

Scope The report will begin by discussing the materials used in invisible cloak, including the description of the materials and also the behaviour of the materials, the design method and the limitation or weakness faced by the experts to develop the efficient invisible cloak.

Collected Data
Invisible Cloak- The Mechanism In order to create an invisible cloak, it is good to know how the invisible cloak works. The main idea of this mechanism is by controlling the light that goes toward the certain object because light works in space differently. Moreover, light contain heat energy and also electromagnetic energy. Thus, an object can be said visible when a ray is direct exactly or reflected at the same angle on the ground plane to the object (Zhang et al., 2011). Meanwhile, an object is invisible when the object that is reflected by the light ray absorbed the light energy or by making a hole into it, thus create the invisibility or so called scattering cancellation method (Su et al., 2013) or the light is stray out of the electromagnetic space (Zhang et al., 2011).

Metamaterials According to Valev (2014) it is a human made materials and it is not exist in nature. It consists of nanoscale building block where the particle allows the light to flow through it and controlled them. One of the methods that physicists and engineers developed is the Forward Design method which is, Scattering Cancellation.

Figure 1. Theoretical model of scattering cancellation method. Sources: Su et al., 2013 From the figure 1, it shows there are three kind of electromagnetic field, such as incident field, and two types of scattered field. Hence, from this figure, the round shape circle that contain negativity permittivity able to cancel the scattering from its internal opposite dipoles object. With the help of isotropic plasmonic metamaterials, the body may induce an intense drop in the scattering section and thus, the object is unseen close with their plasma resonance.

Lens (Optic) Optical Lenses are optical components designed to focus or diverge light. Optical Lenses, which may consist of a single or multiple elements, are used in a wide variety of applications from microscopy to laser processing. According to Professor Howell from University of Rochester (2014), there a lot of cloaking design but the fundamental idea is how to bring to the light to the object and flow through it. Hence, they have developed a method where four normal lenses, known as Perfect Paraxial method to keep the object unseen when the view moves a certain degree away from that optimal viewing position. Furthermore, to make the background of the object undisturbed, the power of the lens and also type of the lens have to be determined. Professor Howell (2014) also claimed that his cloak can be enlarged as the size of the lens and as a result, make the object to be cloaked.

Weakness or Limitation of these method
I. Scattering Cancelation Method - This method however only can be used to a certain dielectric objects and it is impossible to cloak an objects which is larger the one wavelength. Moreover, singular-transformation based cloaks are still inadequate by the superluminal propagation metamaterials. Plus, the bandwidth limitation and polarization sensitivity are still become the main challenge (Su et al., 2013).
II. Perfect Paraxial Method – When the light enters the cloaking devices, it will bend it and pushes it to the centre of the object and probably will make the side axis of the region become cloak or uncloak (Howell, 2014)

Conclusion
Summary
This paper has examined what is the invisibility is, the materials used for the system, the selected design or method to implement it and also some of the discussion about the weakness or limitation when these method are being applied. Moreover, the materials used for the system must have these criteria such as, magnetic response, anisotropy and high gradient refractive index (Gbur, 2103). Hence, with all these two method, hopefully would give a better future for any new research regarding the invisibility.

Recommendation To able the invisibility works efficiently, it is good to have several of method either inverse or forward design so that the expert would have a lot of references to overcome any problems or challenges that they are facing now. More study and research need to be done as well so that invisible cloak would not only rely on these two type materials only. To conclude, the more the studies of the invisible cloak being done, the closer we are to the invisibleness.