USC
About this Article
Written by: Emily Sylvester
Written on: June 27th, 2011
Tags: electrical engineering, ergonomics, material science, physics
Thumbnail by: Oleg Alexandrov/Wikimedia Commons
About the Author
Emily Sylvester was a junior in Aerospace Engineering at the time of writing this article. She was also a member of Society of Women Engineers (SWE) and played for the USC Women’s Ultimate team. In fall 2011, she started working as a Freshman Academy Coach for students entering the Viterbi School of Engineering. As a Harry Potter fan, she was very intrigued and excited by the idea of invisibility cloaks.
Stay Connected

Volume XIII Issue II > The Prospects of Invisibility Cloaks: Bending the Laws of Light
Current research and experimentation with metamaterials have led to advancements in the development of invisibility. Metamaterials can be used to make objects appear invisible by bending light around those objects through refraction instead of away from those objects by reflection. Though no natural material exhibits this behavior, engineers are working to design cloaking devices with metamaterials that will refract light around an object and towards the viewer in the same path as if the object were not present. The engineering breakthrough of manipulating the properties of light to create the appearance of invisibility may ultimately have significant future applications.

Introduction

Crazed American fans, from preteens to adults, have read the Harry Potter books, watched the movies, and have undoubtedly been left with the feeling of awe toward the mystical Invisibility Cloak. Though this fabric seems too magical to be real, science is making it possible. As with most scientific discoveries, time will be needed to take the proof of concept to the final goal of cloaking objects so that they are invisible to the human eye. To understand the developing technology of invisibility, a change of perspective is needed. Rather than making an object invisible, everything behind the object can be thought of as visible as though the object were not there to impede the view. Engineers are synthesizing materials that can bend light around whatever object they encompass, creating the illusion that the object is not actually there, thereby making it seem invisible.

How Light Bends

Tierlieb/English language Wikipedia
Figure 1: Mirrors can be used to create an illusion of invisibility. Through the use of mirrors in this figure, the red animal is not visible to the purple animal.
The nature of light has always been a puzzling concept for physicists. Light exhibits the properties of both waves and particles. It travels in straight lines, like an unaccelerated particle; yet, it exhibits diffraction patterns as do water and sound waves. This dichotomy is called the wave-particle duality. It is the wave properties of light that make cloaking possible. When light interacts with an object (a wall, mirror, or even air), it reflects and refracts. Light reflects from a mirror like a ball bouncing off a wall: the incident and reflected rays make equal angles with the normal line (perpendicular line) to the reflecting surface. We can use this understanding to direct light along precise paths. Lasers and holograms make use of this simple law of reflection. It is the more complicated law of refraction, however, that describes the bending of light. Recent advances in the understanding of this rule allow for the prospect of engineered invisibility. Bending light around what is usually blocked by an object, without reflection, would allow the viewer to see what is behind the object instead of the object itself, thus creating the illusion of invisibility (see Fig. 1).
Oleg Alexandrov/Wikimedia​ Commons
Figure 2: Due to Snell's law, when waves pass through a region of a low index of refraction (the top region) and then hit a material with a higher index of refraction (the grey line), the waves that pass through the material will bend (in the lower region).
The laws of physics govern the bending of light, and an engineer can only make light go where the laws of physics allow. The rule that governs the path of light rays is Snell's Law, which says that light travelling through a single material (e.g. air, water, glass) travels in straight lines, and when light travels from one material into another, the rays bends (see Fig. 2). Snell's Law provides a specific formula for how much light bends in terms of the two media’s indices of refraction, a property specific to each substance.
The index of refraction can be thought of as the slowness of the speed of light in the medium. Its technical definition is the ratio of the speed of light in a vacuum to the speed of light in the medium. The larger the index of refraction, the slower that light travels. Variations in the index of refraction lead to incorrect views of objects. For instance, a mirage is formed when hot air near the ground has a lower index of refraction than cooler air above, causing the light rays coming down from the sky to bend upward as they pass through the hotter air. By the time the light rays reach human eyes, they have bent so much that they are moving up rather than down. Human eyes are not trained to interpret the bending of light, so the image of the sky in the ground is seen instead of the ground itself. These natural variations in index of refraction bend light in a way that makes the sky visible in a place where it is normally not seen. No known natural materials can perform the opposite phenomenon, that is making something invisible where it should be visible. This feat requires a material with a negative index of refraction. The secret behind this phenomenon lies in mathematics.