Invisibility
Invisibility is the state in which an object, a substance or a radiation to the human or animal eye is not perceptible. The invisibility in the strict sense, there are physical environmental conditions under which a normally visible object for people is no longer recognizable.
- 3.1 Invisibility with photonic crystals
- 3.2 Invisibility with mirrors 3.2.1 plane mirror arrangement
- 3.2.2 parabolic mirror arrangement
Importance
Invisibility is of relatively great importance in many fields. For predator and prey, ie in animals both in the animal as well as in the plant kingdom, it is an important factor to itself in the sense of conservation to be successful.
For humans, the topic is important in the field of espionage, the military, astronomy, behavioral sciences, philosophy and the arts. Instigated from these areas of interest, the physics, chemistry and other disciplines deal with it. General brings an understanding of scenarios in which an environment state is not currently perceived, an understanding of where the boundaries of this perception gap are found and how these limitations can be circumvented. There remains one hand, the interest, to intentionally cause such a state of relative invisibility and on the other hand also can expose accordingly invisible things specifically.
Simple examples
For the man and his perception applies:
Examples 5 and 6 for the invisibility based mainly on the resolution of the human eye by about one millimeter size at 3.5 m distance ( angle of one arc minute).
Transparent objects, which are embedded in liquids with a similar refractive index to be invisible. When used in the immersion microscope glass surfaces is covered with oil in order to reduce the interfacial reflection for this purpose. In nature there are many transparent creatures in the water, where they are largely invisible. If you take these creatures out of the water, then they are clearly visible, because the refractive index of air is significantly different from that of water.
Another cause of invisibility is too low brightness, such as, inter alia, during the night (see also Example 6). Even in an environment that is filled with a light-scattering fog or smoke, objects are invisible when they are outside of the then reduced field of view.
Concealment
If this object is behind another object, then it is invisible to the viewer, but one then sees the other object in front of it. An intelligent observer who is present in the scenario, so it will possibly be a concern, even to him currently hidden areas to inspect sooner or later, unless the covering object for him somehow are apparent reason to do so. There is the possibility that it is hidden only from one direction, for several or even out of all. The invisibility is not performed by a property of the object but rather induced by the properties of the covering article. For example, a covered tin their contents in all directions. Painted this one can now black and presents them as a black background the can and its contents becomes invisible. It was merely manipulated the can while the content has not been touched.
Indirect perception
To make an object invisible, often it is not enough to prevent light from this object reaches the observer. Even if the object is completely black and that no light reaches the observer of it, it still covers part of the background. If this is not as completely black, the object by its contour is noticeable.
If an object has exactly the same color and brightness as the surrounding landscape and its background, it is still invisible.
Typical real backgrounds are structured and change their properties with time, such as the angle of view of a moving observer or, depending on time of day and light. A military camouflage or even a camouflage attempts a specific scenario, such as Forest, desert or stone landscapes to replicate in his appearance. Here, a more or less volatile viewer is a reduced likelihood the contour of the respective carrier can no longer be separated from the environment impression. This probability decreases when object and background move towards each other and the observer is able to identify independent motion fields from the scenario.
Some representatives of the molluscs ( mollusks ) have mastered the methods of camouflage better than humans. Octopuses can change the look of the background better than simulate any human camouflage. The gelatinous squid is almost transparent. The weapons Kalmar has light organs on the underside that can outshine its dark silhouette against the water surface.
Technical and Physical concepts for invisibility
Invisibility with photonic crystals
A theoretically conceivable possibility of an all- invisibility could be based on the basis of refraction - reflection - refraction. A hollow ball of photonic crystals ( metamaterials ) and materials with negative refractive index ( left-handed materials ) with the usual for these exotic refraction and reflection behavior could redirect ordered coming from all directions of space light. The hollow ball made of photonic crystals from all directions should have the same optical properties, and the light beams may cross over one another without hindrance, because the photons associated with the bosons. In place of the hollow sphere could use a hollow cylinder with a vertical axis for the first trials, which would then be visible from all horizontal directions of view.
In October 2006, scientists for the first time to make a stealth. In this case, microwaves are directed around a ring, consisting of several layers of copper wires and glass fiber sheets, so that the non-reflected radiation and the respective electromagnetic field appeared partially invisible. However, visible light could not be distracted. The method currently only works on one level and not with three-dimensional objects.
2010, researchers were able to create a flexible polymeric film, a kind of metamaterial also deflects visible wavelengths around 620 nm.
Invisibility with mirrors
Much easier than the all- invisibility can the invisibility for only a glance towards realizing credible, at least for a human observer. The light beams of a certain field of view are directed to the object. Since, however, easily change the distance by the diversion, is a perfect identity unreachable. Likewise also always some remains of the mirror edges remain recognizable. The observation point is flexible within certain limits.
Plane mirror arrangement
In this application, it is advantageous if the mirror, similar to that of overhead projectors are mirrored to the front. This avoids disturbing multiple reflections on the glass surface. For a laterally correct image is always an even number of mirrors required ( this rule applies only to plane mirror ).
The problem with invisibility with mirrors is that the mirror you have to arrange so that they are hard to see. When using four flat mirrors on the nature of a double periscope is still a plane mirror from the back to see. When using three plane mirrors is the image of the background on the head.
Invisibility with mirrors, principle
Optimization of 4- mirror system
Invisibility device, three views
With four flat mirrors can direct the image of the background to the invisible to be made subject in a zig -zag around, with two invisible zones arise. The required area of a 4- mirror system is at its lowest when the two inclined mirrors (seen in the drawings left and right) inclined at 30 degrees to the light beams. The two auxiliary mirror (seen in the drawings above and below) must always be aligned parallel to the light rays, and should be as thin as possible. More information can be found in the image description the optimization of the four - mirror system.
Parabolic mirror arrangement
With two parabolic cylindrical mirrors and a small plane mirror, the image of the background around an object can be passed so that it seems to disappear, at least in one direction. Large parabolic cylindrical mirrors are relatively expensive, and the overall system is strongly dependent on viewing angle. The lying in the focal line of the parabolic cylinder mirror plane mirror, and his two supports should be as thin as possible.
Invisibility with cylindrical lenses
Invisibility with cylindrical lenses
Invisibility with Fresnel cylindrical lens
Invisibility with lentils
Since large lenses are required for the Invisible - making, you can use Fresnel lenses to save weight. A cylindrical lens is composed of partial surfaces of a cylinder jacket, and focuses the light to a focal line. In this focal line is a narrow plane mirror reflects the light to a second Fresnel cylindrical lens. In order to avoid two separate invisible zones, two halved cylindrical Fresnel lenses may be used.
In contrast to other methods of invisibility, this method is relatively little space. All parts are the wall of a cuboid, and the half of the interior is visible. Again, that from a certain point of view, only the background of the object but not the object itself can be seen.
For the technical realization: the image coming from the rear light passes through the focal line below the front image. A small part of the front image you see at the top of the screen. This is a consequence of the spherical (here cylindrical ) aberration. This could be corrected by the circumcision of the lenses.
Invisibility by gravity
Strong gravitational fields, as they occur in the vicinity of neutron stars and black holes can deflect the light so strong that the causative celestial body is invisible. On Earth, therefore you can not produce such strong gravitational fields, because you would have to compress huge crowds to extremely high density.
If this should succeed in distant future yet, then you would have produced a by its strong gravitational effect and by its Hawking radiation very dangerous small black hole. In a harmless way, the gravitational field of our sun ( " the effect of gravity on light " see drawing ) changes the apparent positions of neighboring stars. So you could say exaggerated, that these stars are invisible at its actual location. This is also true for other phenomena astronomy, for example, by the aberration and the Astronomical Refraction.
Invisibility with micro camera projectors
The transmission of the background picture to the front of an object leading to its invisibility.
It was also made the suggestion that it sets up in front of the invisible to be made the subject of a flat panel display with high luminance, and behind the object a color video camera that transmits the image of the background to the screen. It is clear that in bright sunlight the screen would have great problems with the luminance.
In differentiated wallpapers then arises the question on the perspective, and also whether more wide-angle or telephoto lens would be good for the camera. Of course you could pull on a high resolution screen with direction-selective micro - ball lenses, but this would lead to higher costs. The goal of such action is to provide any angle with the matching wallpaper. An already realized example of these are the stereo images that are created with micro -cylinder lens.
The invisibility with micro-cameras who are also micro- projectors:
The twelve blue circles represent the micro camera projectors, but in reality some hundreds of thousands of them are needed. The black lines represent the external light beams represents the green lines inside represent the correlation of the image signals represent, but by no means a wire that connects the camera projectors.
The micro projectors camera should have plano-convex lenses which are curved outwards and inwards flat because outwardly curved lenses would interfere with each other at grazing incidence.
This technology is not currently feasible, but we try to attribute it yet: The resolution of the human eye is at 10 m distance 3 mm, and our system will at distances greater than 10 m function. It follows that the micro camera projectors may be 3 mm. The invisible object to be rendered will initially be a square of 90 cm side length. It follows that 900/3 = 300 camera projectors per screen line needed, and a total of 300 × 300 = 90,000 must use camera projectors. Within the camera projectors have 300 CCD-LED-P Ixel total of 3 mm square, they must therefore be 3/300 = 0.01 mm in size, but must be three-colored. This means that on an area of 0.01 mm x 0.01 mm must exist six areas, three CCD sensors and three LED - illuminated areas, each for red, green, and blue. This can be produced easily, as used in digital cameras already with today's chip technology.
Thus, the light from the LED - illuminated areas can not reach and disrupt the CCD sensors, one could build interfaces between them, but for reasons of space, due to the light diffraction, light scattering, and the interface reflection would not be a good method. Better would be a temporal separation. 1/20 second = 50 ms ( milliseconds) (although this has the disadvantage that the object has its own shadow, you can not see with sufficient intensity of the LEDs, back there, if you look through the object on it. ) 50 ms = 20 ms 5 ms CCD active break 20 ms 5 ms LED active break, and over again. If you want to perform any temporal separation, for technical reasons, but wants to work continuously, then you could order the micro-cameras and the micro projectors side by side in separate micro-packages. Ideal would be a hemispherical shape for the imaging plane of the camera projectors. However, there are semiconductor chips from single crystals that are suitable only for flat surfaces. As a compromise could be to choose the form of a cube which is cut in half parallel to two of its opposing faces.
A big problem is the luminance dar. Each pixel in charge of a parallel light beam of 3 mm diameter. This means that the luminance of each pixel should be 90,000-fold as high as the luminance in the light beam. In the view looking towards the sun, the CCD sensors are likely to be damaged and the LED - illuminated areas will not be able to prevent you from casting a shadow in the sun.
In the distant future could solve these problems with photonics. Even today, there are optical fibers that can actively enhance the incoming light by the laser effect. Then you have only the small problem of how stuffs 8.1 billion each about a meter long optical fibers in the interior of 90 cm diameter. Correct connection of optical fibers should be carried out by micro- robots. In principle, the green wiring diagram applies here inside the image above. In all of this we should not forget that you also need space for the power supply and the passenger, because otherwise the appliance only makes itself invisible.
Culture and Philosophy
Abstract concepts such as feelings and thoughts, such as infinity, ie things that you can do a pictorial idea is not or only by metaphors for the human eye invisible phenomena.
In Greek mythology, the gods were invisible walk the earth, just as the ghosts. In Germanic legends make the dwarf Alberich kings and Laurin invisible using a cloak of invisibility.
The philosopher Plato distinguished between the outer surface of things and their abstracted essence, the ideas.
The abstract painting tries invisible abstract, such as mental, to visualize structures, concept art works with the invisible meanings behind the visual surfaces.
Invisibility in literature and film
Invisibility is an attractive topic of sci-fi and fantasy culture since time immemorial. Best known here is HG Wells novel The Invisible Man, who often acts as the basis for other processing of the topic. The Invisible Man was a popular TV series with an invisible hero. Invisible Girl is a member of the superhero group The Fantastic Four. Stories of invisible people often also address the moral decay of these, such as the ring bearer in The Lord of the Rings by JRR Tolkien. In 1996 a British children's series called Invisible.