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As far as movies are concerned, invisibility is a trope that has consistently fascinated viewers all around the world, myself included. The concept of invisibility was first popularized by Star Trek in the form of cloaking devices for spaceships, before it was later co-opted by other films such as Lord of the Rings and Harry Potter, among others.
While the idea of turning invisible is undoubtedly intriguing, the science behind achieving actual invisibility hasn’t yet been entirely figured out. One of the main reasons why invisibility has boggled scientists for so long has been the complexity of light.
Under normal circumstances, when any material is strafed with light of any wavelength, it typically reflects or absorbs it. If light is reflected, it would bounce and return, allowing the person to observe the object directly. And if light is absorbed, any background light and “signals” would be obscured, also alerting the person to the object’s presence. In either scenario, the object won’t appear invisible.
The only way to achieve invisibility would be to invent a way for light coming from behind the object to somehow still arrive on the same trajectory, as though it is transmitted directly through the object.
During Bilibili’s inaugural Super Science Night event held on October 28 in Beijing, Chu Junhao, an academician from the Chinese Academy of Sciences, introduced a novel approach of achieving invisibility. During his demonstration, Chu’s assistants unveiled a thin plastic-like sheet that has been designed using an approach known as “lenticular grating.”
Lenticular grating operates on the principles of wave optics which, in simple terms, refers to the study of the behavior of light waves—understanding how they travel and interact with the world around them.
This approach involves the application of an optical component or material comprising an array of tiny cylindrical convex lenses, arranged on the surface of the ”invisibility cloak.” Depending on the arrangement of these lenses, and the distance between the object and the cloak, light is refracted differently, enabling its manipulation to achieve a variety of optical effects, such as bending light rays, refracting light in specific directions, and creating dead spots.
In the context of Chu’s demonstration, the sheet consists of thin lenses arranged tightly in parallel rows. Each lens refracts incoming light, and when a significant number of these lenses are compressed together, thereby forming the sheet, light that travels to and fro the object through the sheet gets distorted into countless, identical thin strips. At certain angles, the object will seem like it has disappeared, achieving an invisibility effect.
“In the future, the invisible cloak from the Harry Potter series may become our daily outfit,” said Chu during his demonstration.
While lenticular grating could eventually turn invisibility cloaking technology into reality, limitations exist. In particular, it’s crucial to recognize that both lens arrangement and object distance are critical factors to ensuring the effectiveness of lenticular grating. Again, in Chu’s case, changing the orientation of the sheet, or moving the object (which in this case, is Chu) further away from it, will likely dispel the illusion of invisibility.
Instead of relying on carefully arranged arrays of lenses to achieve the desired effect, another approach currently being studied by researchers around the world is the use of metamaterials.
Materials interact with electromagnetic waves—visible light included—based on their refractive indices and intrinsic properties. Metamaterials refer to a niche category of materials that exhibit a negative refractive index, enabling scientists to manipulate light on scales smaller than its wavelength. What this essentially means is that these materials can bend lights in ways more versatile than and normally unachievable with natural substances, such as those used to construct the sheet showcased during Chu’s demo.
However, metamaterials come with their own set of challenges. Due to their unique property, metamaterials generally need to be crafted at the tiniest scale possible, making the creation of any structure with metamaterials either a rigorous process that requires precision and expertise, or if not, outright impossible (at the moment).
Visible light also falls within a specific range of the electromagnetic spectrum, with wavelengths ranging from 400–700 nanometers. While metamaterials have the potential to manipulate visible light effectively, such manipulations remain bound by scientific principles. These principles dictate that light waves become harder to manipulate as the wavelength decreases, due to precision-related limitations. Other constraints, such as chromatic aberrations, light absorption, and light scattering, also need to be effectively addressed before metamaterials can be utilized for the advancement of invisibility cloaking technology.
We are likely still a decade or more away from witnessing the commercialization of invisibility cloaks. However, if sci-fi movies are any indication, perfecting the technology, as seen in the Star Trek series, took centuries. If anything, our current progress suggests that we may achieve feasibility in invisibility cloaking technology ahead of schedule.