Invisibility Cloak

8 min read

Harry Potter’s invisibility cloak (Image credit: Warmer Bros. Pictures)

Harry Potter introduced many of us to the idea of an invisibility cloak, a piece of cloth that is able to completely hide anything and anyone hiding within it, no matter where it is seen from. The cloak has the ability to seamlessly blend into its surroundings, producing perfect invisibility.

Could the cloak be a form of technology we have right now or perhaps achievable in the near future? In this article, let us investigate three methods where we can tinker with the concept of invisibility and whether Harry would have a better chance at fighting Voldemort if he were to have them. 

First, let’s explore the requirements for this effect:

  1. Remain unseen no matter the viewing angle
  2. Clear and highly detailed image of the background

Projection Mapping

There are a number of techniques that can be employed to create the illusion of invisibility. In a classroom, projectors are used to show an image on a projection screen. When a teacher walks in between them, do you notice the image being projected on them? This is the very basis of this technology!

You may have heard of projection mapping, where an image or video is displayed on a complex surface such as the facade of a building or body of a vehicle. By using similar techniques, researchers at Tachi Laboratory from the University of Tokyo have created a proof of concept for their invisibility cloak. A video camera is placed behind the user (the person who is going to become “invisible”) which records the background. The user then wears the “invisibility cloak” which is made from a highly reflective material. The video is then projected onto the cloak, showing the background. This creates the illusion of the invisibility as light appears to go through the cloak. Check out the picture below to see the effect!

If Harry were to use this invisible cloak, it would work quite well if Voldemort fails to notice the projector shining onto the cloak. In fact, if Voldemort stays in a single location, Harry might be able to stay as invisible as with his original cloak! He would even be able to slowly get closer and give him a chance to spring a surprise attack.

This ‘invisibility’ only meets our second requirement as projectors can provide high resolution videos depending on the source. However, the illusion is easily undone as the projector can only project the image in one direction. Viewing from another angle would immediately break the illusion–like seeing a teacher’s shadow on the projection screen. One way to get around this is to have multiple projectors in multiple locations, but that would be very impractical at that point. Anyone passing through would see the many projectors and know something is going on.

Lenticular Lens

Lenticular printing – Image Credit: Wikipedia

Remember those holographic cards and book covers that show different pictures when you tilt them left and right? Those are made using lenticular lenses, which is our next method of ‘invisibility’. The lenticular lens is a piece of clear plastic with small ridges that resemble vertical bumps. Other than being able to recreate the illusion of two images on a single surface, it can also give us the illusion of invisibility!

The lenticular lens produces ‘invisibility’ by manipulating the light that we see. When a beam of light hits the lens, the ridges will redirect them from their original path. This new path can be calculated using a formula known as Snell’s Law. 

The diagram above shows how, at each portion of the lens, light from across the background is refracted by it to an observer. The image that we see is an average of the surrounding. We can compare this effect to a few drops of red paint in a white paint can. When stirred well, the eventual colour will appear white with barely a tint of red. Similarly, the lens shows how the horizontal image would look stretched out if it were mashed together horizontally!

If Harry were to carry around this invisibility shield, he would have to make sure his background has a pattern that is horizontally similar. If Harry doesn’t move around too much, this could be an easy method for fooling Voldemort into not noticing Harry!

Camouflage technology companies such as Hyperstealth, have been researching the use of lenticular lens in real world scenarios. For example, they have made a product which they have coined “Quantum Stealth”. Essentially, their product uses lenticular lens in different configurations. By stacking sheets on top of each other, different configurations can be achieved that provide functions such as providing more background detail or a repeating image. Who knows, it could become a product we can easily buy in the future. However, going by our definition of invisibility, lenticular lens does not fully meet either requirements. It can only be viewed from the direction the lens is facing and the resulting background would be highly blurry. If used in an environment with many different shapes and colour, it would not work as well.

Rochester Cloak

The Rochester cloak is an illusion that creates invisibility in a set area and from in front of the lens. Researchers at the University of Rochester use four such lenses arranged specifically to bend light such that it focuses itself at the centre of that array. These lenses can be found in optical stores and can be recreated with relative ease, try it out for yourself here!

With the lens in place, light that enters it will go through a series of refractions that flip and adjust the image before exiting at lens 4 in its original state. At 2 points of these refractions, the light crosses at a single point. This results in a doughnut shaped area around the point that is effectively invisible as long as the centre focus point is not crossed.  

Unlike our previous methods, this is much more complicated for Harry. He would require a series of gigantic lens that could hide his whole body instead of the smaller setup we have seen above. However, if Voldemort doesn’t suspect anything is going on, Harry would have a big advantage as he can achieve total invisibility!

As this does not require complex equipment, it could be applied to many scenarios. One basic application could be having the concept applied to a truck to allow drivers to see through blind spots on their vehicles, making it safer for drivers and other road users. So how well does the Rochester cloak hold up with our requirements? It gives a very clear image as there is minimal manipulation of light. However, it is limited by a specific viewpoint where the four lenses are aligned. It would not be a practical invisibility cloak but is definitely an inspiration for low-tech methods!

Currently, we are still decades away from an actual invisibility cloak, but progress is being made every day as scientists try to manipulate the properties of light. All three mentioned methods provide us interesting insight into how it could be achieved. Projection mapping is very dependent on high tech equipment to give a convincing illusion while the lenticular lens and Rochester cloak play with the bouncing of light. It is but a matter of time before true invisibility is possible, maybe in our lifetime!

Written by Oh Wei Sian
Illustrations by Jasreel Tan


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