How To Achieve Optical Invisibility?

I have seen the design of many concept mobile phones, which are probably circuit boards constructed of invisible materials.

But I think now that this kind of technology is available, it must be used for military purposes first, to create optically invisible robots for spying and other spy work, right?

Let me start with the conclusion: there are several simple and effective solutions to invisibility in a specific direction. Invisible in all directions in 360 degrees, theoretically possible, but no one can do it yet.

The more orthodox optical stealth method is nothing more than two points: (1) The strong absorbing material is used as the shell, so that the radar cannot detect it; (2) The metamaterial with negative refractive index, and the transformation optical structure designed based on this metamaterial, Let the light bend, the basic principle is very simple, as shown in the figure below:

The research to reduce the radar cross-section° is relatively mature and has been applied in actual combat for many years. There are three commonly used stealth methods: shape design technology, absorbing material technology, and cancellation technology. The molecular components of absorbing coatings are relatively diverse, such as ferrite absorbing coatings, tetrapod-shaped zinc oxide and so on. The thinner the coating, the wider the absorption spectrum and the greater the attenuation of the reflected wave, the better the absorption effect.

(1) One is proposed by a Japanese scholar. The back of the object is imaged with detection, and then the image is displayed on the front of the object with a color LED screen. In this way, from the perspective of the observer, the back of the object is directly seen, as if the object does not exist. In Germany, an engineer wrapped up a Mercedes-Benz car in accordance with this principle, and ran on the streets of Germany during the day and night. The passers-by were completely messed up and captured all kinds of photos. See this article:

This stealth method of letting objects blend into the background is rough, but it has the most practical application value. The reason is the same as that of camouflage uniforms, and it is not easy to be found by the opponent.

(2) Let's talk about the gameplay of the University of Rochester. Professor John Howell of the University of Rochester was originally focused on quantum physics. The large glass bucket of water has realized the invisibility of the large object. Extremely simple and outrageous.

Later, his doctoral student Joe Choi made an invisible system with four of the most inconspicuous ordinary lenses. See picture: