Have you ever pondered about the physics of invisibility: the physics behind how we could possibly make someone invisible? Perhaps you picture bending light around someone to shield them with invisibility. In this article we will discuss the physics behind an invisibility shield. The physics of invisibility come to be interesting and oddly simple in this case, let us begin.
Firstly, we need to establish a basis for what is meant by invisibility. The Webster Dictionary defines invisibility as: incapable by nature of being seen; not perceptible by vision. Consider an object you want to make invisible. How large/small is this object? Will it be invisible from all angles? What will people see when they attempt to look at the object? Thinking about invisibility is an exciting and challenging mental exercise. There is so much to consider and fortunately there are already designs coming to the market. We will discuss the physics of this technology while also touching on metamaterials as a topic.
The Invisibility Shield
There is an invisibility shield currently in development and being kickstarted by Invisibility Shield Co. They tested the shield through various lens shapes, profiles, angles, and separation distances. After development the results are impressive because invisibility is achieved.
The entrepreneurs behind the shield themselves cite key features in their Kickstarter pitch. These features include high resolution invisibility in a thickness of 2.3 inches. The shield weighs less than 9lbs and is fully recyclable. The structure of this metamaterial is an array of lenses much like the cylindrical lens shown below. These lenses are basic convex lenses used in varying applications from laser focusing to clarifying the image of a telescope.
These lenses are extruded such that they become a long rod with one flat edge and one curved side. These rods create an array of one rod right next to another while vertically oriented.
Physics of Invisibility
Now that we know the structure, how does this work exactly? Well to begin we must consider how its components work. We will get a closer look at one of the lenses provided by the company behind the Invisibility Shield. Take note of the path of light through this convex lens.
Shown above is the path light takes through one of the invisibility shield’s lenses. Note the internal reflection towards the edges of the lens. If we recall from Snell’s law we can easily explain this behavior.
Total internal reflection occurs when two conditions are met. One is that light is traveling from a dense medium to a less dense medium. The other is that the angle of incidence be greater than the “critical angle”. This critical angle is the θ1 that corresponds to when θ2 is 90°.
Light Paths Used for Invisibility
In the case of our lens let us consider the parallel light entering it. This parallel light must then exit through the curved surface of the lens. The angle of incidence changes at different points along the curvature of the lens. Eventually there is a portion of the lens which will experience internal reflection as shown in the figure above.
The rest of the lens works as you would expect, focusing light to a point. The focal point of these lenses is very close to the shield. Because of this, by the time light reaches an observer a subject’s image is made extremely diffused. The image below demonstrates the final path that light takes. A combination of diffusing and reflecting makes it so that some of the light from the subject does not even make it to the observer.
As a result this shield effectively is replacing the image of what is directly behind it with light from its surroundings. Also note that due to the construction of this there are some limitations. For one this construction diffuses light horizontally. In other words if someone were lying horizontally behind this shield it would be less effective. Given that this is a shield it only currently provides invisibility from an observer in one direction. It is nonetheless an innovative demonstration of human ingenuity.
Metamaterials Designed with Physics of Invisibility
As mentioned before, the invisibility shield is really a metamaterial. A metamaterial is an engineered material which has properties not found in naturally occurring materials. In this case the metamaterial produces an array of focal points in front of itself. The array diffuses light from directly behind the shield. This effectively replaces the image of what one would expect to be directly behind the shield. the subject’s image gets replaced with background surroundings.
Metamaterials have been around since the turn of the 19th century. They have been utilized in a variety of applications such as antennas, sound filters, and even as seismic protections. One notable branch of this technology is negative refraction index metamaterials. The notability of this branch comes from the new behavior seen in unorthodox interactions with light.
We have touched on some very basic physics in our discussion. It simply goes to show that innovation shines even with basic principles. The true take away from this little discussion is to show how we are progressing in all sorts of technologies. The invisibility shield demonstrates one of many metamaterials currently used to produce invisibility. Another notable invisibility invention is Hyperstealth Biotechnology’s “invisibility cloak”.
This product is another interesting case of invisibility and shows what we can do with metamaterials. As shown above the metamaterial from Hyperstealth Biotechnology other kinds of invisibility such as thermal can is also a realm of invisibility to consider. The physics of invisibility will have more breakthroughs with more materials and you can hopefully see more products of invisibility soon.