Wow! I saw an academician of the Chinese Academy of Sciences demonstrate the "invisibility technique" on the spot. What is the scientific principle behind this?

Recently, at a scientific event, Chu Junhao, an academician of the Chinese Academy of Sciences, introduced the "Effect Test of Various Optical Invisibility Methods" and demonstrated the "invisibility technique" achieved using special materials. What is the principle of invisibility during the experiment? How long will it take for invisibility technology to enter our daily lives?

0 1 Uncovering the mystery of the “disappearing lower body”

At the scene, Academician Chu Junhao asked the staff to hold a panel. At first, the lower body of the character could be seen through the panel, but after rotating the material 90 degrees, the lower body of Academician Chu Junhao "disappeared", but the stage scene behind the character could be clearly seen. Academician Chu Junhao said: "In the future, Harry Potter's invisibility cloak will become an everyday item in the wardrobe."

▲ Before and after material rotation

Chen Zheng, associate professor at the School of Physical Science and Engineering of Beijing Jiaotong University, introduced that the principle by which Academician Chu Junhao achieved the "invisibility" of his lower body is lenticular grating. This material is very special: it is composed of rows of tiny cylindrical convex lenses. Each cylindrical convex lens will compress the object behind it into a thin strip. When there are enough lenses, the object will be decomposed into countless identical thin strips, making it seem as if it cannot be seen, thus achieving a visual "invisibility" effect.

▲Lenticular grating taken with a mobile phone at macro distance

In the experiment, the film plate was rotated 90 degrees to make the cylindrical mirror parallel to Academician Chu Junhao's legs. The reason why the stage background can be seen is mainly because of the horizontal stripes in the background. These horizontal stripes are still complete stripes after imaging, so they are more eye-catching. It is these horizontal stripes that are enough to attract attention, making the stealth effect more prominent. In addition, Academician Chu Junhao also demonstrated other stealth methods, including using two materials with similar refractive indices and plane mirror reflection imaging.

0 2 Perfect invisibility? How do metamaterials achieve this?

Chen Zheng introduced that in a broad sense, stealth means blending into the background. From this perspective, there are actually many ways to achieve stealth within the visible light range. For example, lying in the grass and wearing camouflage clothes is a relatively rough form of stealth. From an optical point of view, to achieve the invisibility of opaque objects, you can use the reflection of a plane mirror to allow light to bypass the object and keep the direction of the incident light consistent with the outgoing light. For transparent objects, the refractive index can be used to achieve invisibility. To achieve true perfect invisibility, metamaterials provide a new solution. Yuan Lanfeng, an associate researcher at the University of Science and Technology of China, introduced that the properties of metamaterials lie in the way the nanoparticles in the material are stacked. When wearing clothes made of metamaterials, the light shining on the body is deflected in the opposite direction, and it looks like the light has not passed through any object from a distance. And metamaterials are not limited to optics, they exist in many electromagnetic bands and even in the field of acoustics. Yuan Lanfeng said: "This is a revolutionary idea. It does not rely on the properties of the material itself, but relies on the different stacking of material structures to achieve previously unattainable effects, such as negative refractive index and negative magnetic permeability."

0 3 Is it still far from putting on an invisibility cloak?

The more stealth technology develops, the more advanced detection technology will be. Chen Zheng introduced that neutrinos are small enough, numerous, and have strong penetrating power. - Currently, neutrino detection has received great attention and can detect nuclear submarines and nuclear explosions in every corner of the world;

The objects of gravitational wave detection are all large-scale objects, and only gravitational waves that are strong enough can be detected.

However, no matter how anti-detection technology develops, there will not be an ultimate detection method that can detect all stealth, and of course there will not be a stealth method that cannot be detected by all detection methods. At present, stealth anti-detection methods in the military field, as well as contact lenses and invisible braces in daily life are all used. In the future, simple stealth methods will be used to make the distribution boxes and sewage wells in the park blend into the environment. There is still a lot of room for imagination in this regard. Chen Zheng said that stealth is just a goal. There are many technical means to achieve stealth, and the industries and scientific and technological fields involved are also very broad. Comprehensive progress in materials, machinery, optoelectronics, chemistry and other fields is needed to finally achieve a more ideal stealth effect.

Source: China National Radio