Figure 1: Imaginary Incognito[1]
Stealth, in various film and television works, has always been a frequent science fiction idea. Whether it's the six babies in "Hulu Brothers", the invisible woman in "Fantastic Four", or the invisibility cloak in "Harry Potter". The classic scene is often like this: a certain Doctor Strange or a mad scientist suddenly develops a special potion in the laboratory, and after drinking it, the body "dissolves" and becomes as transparent as air, but the clothes cannot be hidden, so there is a strange scene of a hat and jacket floating in the air.
So is brain-bending stealth really technically achievable?
Refraction of light and distortion of the image
Let's start with the refraction of light. We can see an object with both eyes because the object itself can emit light, or reflect the light emitted by light sources such as sunlight and LED lights, either way, eventually the light from the object travels to the human eye and is perceived, whereas in the same homogeneous medium, the light travels approximately in a straight line, and the viewer will see an object at a distance and near distance along the eye level. But if light travels through two different media, such as water and air, the situation is no longer so simple. Light travels at different speeds in the air and in water, with a faster speed in the air, close to 300,000 kilometers per second, and a relatively slow speed in the water, like a high-speed train and a green train.
In Figure 2, the light from a fish in the water could have traveled straight from underwater to the human eye in a straight line (A-B-C) all the way through the interface between water and air. But the world of light is also "very involuted", always trying to take the most time-saving path from the starting point to the end, so they will choose another route (A-D-C), the "green train" in the water will shorten the journey (AD<AB), and the "high-speed train" in the air will become longer (DC>). BC), but after all, the "high-speed rail" is much faster, and the total time it takes is still shorter, and in such a way that the light is "bent", which is called the refraction of light. Since the refractive index of water is greater than that of air, water is called a light-dense medium, and air is called a photophobic medium, and the viewer instinctively feels that the light travels in a straight line, and sees the image of the fish in A' above the actual position A', so the fisherman must pierce the bottom of the position of the image he sees when he uses a spear fish.
Figure 2: Refraction (left) and total reflection (right) of light between water and air medium. Source: Light Science Workshop/Veer
If the fish happens to be in a relatively hidden position in the water, the angle between the light from the fish and the water surface is small enough, and the light will be fully reflected like a mirror, and cannot be refracted into the air, which is equivalent to getting off the green train at the station, but getting on another green train without getting on the high-speed train. Because the light is trapped in the water, the viewer cannot see the fish at all from a certain angle, and if it is a chopstick, the lower half is in the water, the upper half is exposed to the air, and when viewed from the top, it is bent and warped, and when viewed from the side, it becomes a dislocated two, which is the "fault" of light refraction.
Figure 3: "Two dislocations" pencils under light refraction Source: Light Science Workshop/Veer
Using light refraction to cleverly change the direction of light can achieve a variety of incredible effects, such as the magic of a coin passing through a water bottle, and the secret behind the magic can be found in another video at the end of the article. Similarly, a picture will inexplicably disappear in the water.
This is due to the fact that the light from the image on the paper card needs to pass through multiple media such as air in the small bag, plastic outer skin of the small bag, water and outside air in order to reach the human eye.
There is also a special water tank, which uses a syringe to inject or suck up the water in the diagonal space of the tank, and two different pictures can be switched between each other as if they were "changing faces". A smiling face can become a crying face, a crying face can become a smiling face, and red and blue faces can also change with each other. As shown in the figure below, the "mastermind" behind it is also a total reflection of light.
Video 3: A tank device that allows you to see alternate back and forth between two images
Figure 4: When the diagonal space is filled with air, the viewer sees Figure B due to total reflection, and when the diagonal space is filled with water, the viewer sees Figure A because light travels in a straight line in a homogeneous medium. Source: Drawn by the author
In a painting by Leonardo da Vinci, the Renaissance master Leonardo da Vinci was also hotly discussed. In Leonardo da Vinci's "Salvator Mundi" below, Jesus the Savior is holding a crystal clear transparent ball in his hand, which seems to be "fine" on the surface. However, the point of complaint of the careful observer is that the folds of the clothes behind the transparent ball, which should be dislocated and distorted due to light refraction, appear so intact in the painting that it is incomprehensible.
Figure 5: Leonardo da Vinci's famous painting "Salvator Mundi" suspected of containing an unreasonable light refraction effect Source: Wikipedia
In this regard, some optical professionals have done serious research, using special optical simulation software, three-dimensional modeling, and ray tracing, simulating various solid and hollow spheres of different materials and thicknesses to obtain refracted imaging results, and compare them with the effects in the painting, and also published a paper entitled "On the Optical Accuracy of "Savior" [2]. In the end, it was concluded that in order to achieve a similar effect as in the painting, it was necessary to use a hollow sphere, and the refractive index of the outer wall of the hollow sphere was 1.51714 and the thickness was 1.3 mm. For comparison, the refractive index of air is 1.0003, the refractive index of water is 1.3333, and the refractive index of glass is about 1.5, so it should be a hollow glass sphere in the painting. But was there a way to process such a thin glass ball hundreds of years ago? Even if it was made, it must have been easy to break, how to hold it by hand?
Of course, there are also people who feel that this kind of rigid rational analysis is meaningless, Master da Vinci is not a realistic creation of an everyday figure, the painting is the holy savior Jesus, the twisted and deformed folds of clothes will show a disrespectful attitude, not to mention that Leonardo da Vinci is not only a painter, but also an all-rounder, for various sciences including optics have dabbled, will not be ignorant of the refraction of light, nor can not observe the image distortion of the real glass ball, most of the people do it intentionally, not carelessly make low-level optical mistakes。 Regardless, the painting itself is worth a fortune, and in November 2017 it sold for a whopping $450 million at Christie's in New York.
An optical device that creates the hidden effect of an object
Returning to the main topic of this article, "stealth", the object that can be seen by the human eye is often reflected into the human eye after the incident light hits the surface of the object, and if the incident light is refracted in an appropriate way, after changing direction, it happens to bypass the space occupied by the object, and the viewer will feel that the light is passing through the empty space and directly reaching the human eye, forming a visual stealth effect, which has been proved to be theoretically feasible in optics [3].
Figure 6: Stealth by curved light bypassing the object to be hidden: For the viewer on the right, there is no actual visual difference between the light bypassing the object (left) and passing directly through an empty space (right) [3]
"If you want to do a good job, you must first sharpen your tools", and in order to achieve such an effect in experiments, it is necessary to resort to various optical devices [4]. Magnifying glasses, myopia glasses, telescopes, and lenses in microscopes are all called lenses, which look like a small round lens on the surface, but the function is very powerful, and the light refraction phenomenon that occurs by using the different refractive indices of air and glass can control the convergence and divergence of light, and can also zoom in and out of the image. In the following optical system composed of four different lenses, through the reasonable design of the placement, the viewer in front of the fourth lens will have a blind spot, and can see the background object behind the first lens, but cannot see the object in the blind zone. When several fingers are inserted into the blind spot between the lenses, the viewer does not see the fingers, only a grid of the background [5].
Figure 7: A stealth system consisting of four lenses with a blind zone (orange area) [5]
Figure 8: Viewing effect of the four-lens stealth system [5]
It should be said that this is just a stealth technique within the field of view of the lens, and if viewed from outside the lens, everything is exposed under the nose, although the lens can also be made larger. Not only that, but the viewer is also required to look directly at the lens, and it is easy to "goof" if you look obliquely.
Unlike circular lenses, which can deflect each ray of light from up, down, left, right, and in all directions, the cylindrical lens sees the object as composed of lines parallel to it, and can refract the entire line in a certain direction. If many small cylindrical lenses are arranged in an array, it becomes a cylindrical microlens grating. The cylindrical lensing grating is also a "good hand" for controlling the direction of light, such as the "double painting" that many people have seen This is because the surface of the card is covered with a layer of cylindrical microlens grating, originally the card is printed with a composite containing two pictures, and the red vertical bar part and the green vertical bar part can be projected to two different directions by the grating, and each synthesize a different photo, and if these two directions happen to correspond to the left eye and the right eye of the person, the picture corresponding to the angle of view of the left and right eyes is projected respectively, and the naked-eye stereoscopic display can also be realized without wearing glasses。
Figure 9: Histolinar microlens grating that can refract light in two different directions to the left and right eyes Source: Wikipedia
In addition to the double painting and naked-eye stereoscopic display, the lenticular lenticular grating can also show off stealth [6], which stems from its "discriminatory" treatment of "fat" and "thin" people. For an object in a parallel direction behind the grating, if it is a "thin hemp rod", the light will be easily refracted out of the viewer's field of vision and be hidden, while if the "heart is wide and fat", there will be a large amount of light entering the viewer's field of vision, which does not seem to disappear.
Figure 10: Light refraction effect of cylindrical microlens gratings on objects of different widths Source: Drawn by the author
The slender chopsticks at the bottom mysteriously disappear, but the horizontal stripes on the back are unharmed (due to the large width of the chopsticks in the vertical view). Of course, the direction of the grating must not be wrong, and the chopsticks should be nearly parallel to the direction of each cylindrical lens.
In a popular video on the Internet, academician Chu Junhao, an optical expert, showed at a party that his legs "disappeared" under the cover of a magical material, but the horizontal lines of the stage background behind him can still be clearly seen, and this cylindrical microlens grating is also used. The defects of this stealth method are also obvious, for the shape of the object to be hidden, "tall, short, fat and thin", whether standing or lying, there are strict requirements, as long as the "substandard" will be "exposed", the cylindrical microlens grating is destined to only be used in specific stealth scenes.
The following optical stealth device is even larger: in a large pool[7], a special hexagonal prism-shaped tank is placed, in which only the most central "cave" is flooded with water, many of the rest are hollow and contain only air, and a cross-section in the shape of a six-pointed star (dark blue in the image below) is solid and composed of glass. In this way, the incident light on one side is refracted by the light between the three mediums of water, air and glass, and due to the special geometry of the water tank, it is equivalent to a cylindrical water cavity that can bypass the center, and the viewer on the other side feels that this part is gone. As soon as a fish swims into the hole of the water cavity, it will disappear without a trace, and the viewer will only see the transparent pool water and green aquatic plants behind the tank.
Figure 11: A hexagonal prismatic optical device that hides fish in a pond [7]
[4] Similar "hexagonal castles" do not have to be built underwater, but can also be viewed on a tabletop [4]. It can be found that the metal weights that were originally surrounded by glass prisms are missing, but the car model behind it can still be clearly seen.
Figure 12: Hexagonal prism light stealth device[4]
Negative refractive index materials and the realization of true stealth
Although the above various optical stealth devices can provide a certain degree of object disappearance effect, there is still a big gap between the real stealth suit that everyone expects, and the concept of negative refractive index materials that has emerged in recent years has ignited new hopes for stealth.
Natural materials generally have a refractive index greater than 1, assuming that the refractive index of water is negative, then you will see an image of a fish floating in the air above the surface of the water. More importantly, compared to the mirage effect, the material is an ideal choice to wrap objects to be hidden in it like clothing, achieving the aforementioned vision of bending light and bypassing invisible objects. Since there is no such material in nature, researchers have set their sights on using nanotechnology to synthesize artificially.
The world-famous British Museum displays a magical piece of glass goblet art, known as the Luchigues Cup, made by the ancient Romans 1,600 years ago. The peculiarity of this artwork is that when the light hits from the front, the cup appears green, and when it shines from the back, the cup appears red. The researchers found that this "chameleon" effect was due to the dissolution of gold and silver metal particles into the glass, which were only about 50 nanometers in diameter. Although the ancients at that time did not know what nanomaterials were, they had inadvertently set foot in this technology through repeated practical attempts. Similarly, in many parts of the world, nanoparticles are used in the brightly colored painted motifs on the windows of historic cathedrals.
Today, in the 21st century, researchers can manipulate various types of nanoparticles more freely and flexibly, just like playing a game of Tetris, or building blocks, placing them in different positions, arranging them into different structures, producing various wonderful material properties, and artificially synthesizing new materials with negative refractive index has also become possible.
In a paper published in the journal Science in 2015 [8], the researchers used fluoride of silver, magnesium and other metals to form a network material, and then interspersed the nano-silver wires generated by porous alumina into it.
Figure 13: Effect of a negative refractive index nanomaterial stealth cloak: with and without stealth cloak coverage (left) [8]
Judging from the effect shown in the paper, this stealth cloak is not in vain, but the current results are still observed under the microscope, the size of the stealth cloak is only micron level, thinner than a human hair, and it requires harsh experimental conditions, and it cannot be applied in ordinary life in the short term.
On the one hand, it is not difficult to create an optical trap device to place objects in the blind spot and make them disappear from the front of the eyes through the "blinding method", and "all roads lead to Rome" On the other hand, the real sense of invisibility clothing in science fiction imagination, although theoretically feasible, is not easy to really process and manufacture, negative refractive index materials have brought new hope to researchers, but it will be difficult to appear in practical applications for a while.
bibliography
[1] A. Cho, High-Tech Materials Could Render Objects Invisible, Science 312, 1120-1120 (2006)
[2] M. Liang, M. T. Goodrich, and S. Zhao, On the Optical Accuracy of the Salvator Mundi, arXiv:1912.03416 (2019)
[3] J. B. Pendry, D. Schurig, and D. R. Smith, Controlling Electromagnetic Fields, Science 312, 1780-1782 (2006)
[4] K.-T. Lee, C. Ji, H. Lizuka, and D. Banerjee; Optical cloaking and invisibility: From fiction toward a technological reality, J. Appl. Phys. 129 (23), 231101 (2021).
[5] J. S. Choi and John C. Howell, Paraxial ray optics cloaking, Opt. Express 22(24), 29465-29478 (2014)
[6] Su Yibo, Wang Quan, Cao Feng, Chen Shumei, Deng Bochang, Exploration of the Invisible Phenomenon of Objects by Cylindrical Lens Arrays, Physical Experiment 43(9), 22-28 (2023)
[7] H. Chen, B. Zheng, L. Shen, H. Wang, X. Zhang, N. I. Zheludev and B. Zhang, Ray-optics cloaking devices for large objects in incoherent natural light, Nat Commun 4, 2652 (2013).
[8] X. Ni, Z. J. Wong, M. Mrejen, Y. Wang and X. Zhang, An ultrathin invisibility skin cloak for visible light, Science 349, 1310-1314 (2015)
Written by: Jiao Shuming
Reviewer: Gao Hui
Producer: Zhao Yang
Editor: Zhao Wei
Source: China Optics
Original title: "Curved Light and Stealth"
Edit: Hanamaki