|MadSci Network: Physics|
Greetings: There are at least four ways to attempt to make objects appear to be invisible to an observer: 1) by blocking light from illuminating an object (e.g. darkness); 2) by blocking or redirecting the light reflected from an object away from an observer 3) by redirecting the light reflected from an object to make it appear to the observer to be in a different location than where it actually is located (ghost objects) and 4) to make the object blend into the background (camouflage). Magicians and entertainment parks such as Disneyland make objects appear and disappear in their displays by using these four techniques. Typical optical components used at Disneyland include white light and laser projectors, two way mirrors (e.g. optical beam splitters), lenses, holograms and high quality mirrors. The military make objects invisible by painting them to blend with the background (camouflage) or to make them as mirror like as possible to not reflect energy back to the observer. The mirror like surfaces on the F-117 Night Hawk stealth fighter surface have been faceted to make the aircraft invisible to microwave radar (microwave mirrors) and it is colored black to make it hard to see optically at night (camouflage); also, the hot portions of the engines and exhaust are designed to block the infrared energy from reaching observers on the ground. The Night Hawk web pages are at the following URL: http://www.plweb.at/lockheed/develop.html There is a configuration of planar mirrors called the K-mirror with the edges of 3 planar mirrors forming a K shape. In my illustration below a light ray comes from the left (arrow) and reflects down off tilted mirror M1 (not shown at the correct 22.5 degree tilt angle) and then the ray reflects off of the bottom mirror M3 back up to the tilted mirror M2 (also not shown at the correct tilt angle) and the ray continues to the right toward the observer (O). Thus the light from the backgrown is bent around the object hidden at point (X). The most difficult part of the illusion is to make the observer not see the mirror edges. Magicians are good at this type of detail. M1 M2 \ / ----------> \ X / ---------------> O) \ / M3 __________ Fiber optics could be used in place of the K mirror. They offer a very transparent, flexible, transmission line for light which could also bend light around object (X). Fiber bundles containing thousands of fibers called endoscopes have been designed for medical applications. Endoscopes transfer images from the inside of the human body to a physician for diagnostic and surgical applications. Even though fiber optic endoscopes are only about a meter long they cost thousands of dollars. However, even at the high cost their unique capabilities are finding widespread use in medicine. All of the optical functions that can be performed by beam splitters, mirrors and lenses can also be implemented in fiber optics; however, they are considerably more expensive and often are not as efficient as conventional optical components. The flexibility of fibers and their small size do offer applications such as endoscopes and fiber optic communications links that conventional optics cannot perform. Once again hiding the surfaces of the fibers from view would be very challenging. The third type of invisibility presented at the beginning of this note uses fiber optics to confuse optical and microwave radars with ghost targets. A radar or LIDAR (light radar) transmits a beam of light pulses out many kilometers (miles) and looks for reflections from targets. The distance and speed of the target can be accurately measured by measuring the time it takes for a light pulse to travel to a target and to be reflected back to the LIDAR. If several miles of optical fiber are coiled up in a small volume about the size of a large coffee mug, a compact optical "delay line" is formed. If the target has a lens that focuses a LIDAR pulse into a fiber optic delay line, the light pulse travels several extra miles inside the coffee cup before it is directed from the end of the delay line back toward the LIDAR. To the LIDAR this delayed light pulse looks like a large target several miles farther away from where the actual target is located (a ghost target) and the strength of the ghost target is much greater than the light reflected from the real target. If several fiber optic taps are made along the delay line at different distances, the LIDAR will detect the returns from each of the taps producing several ghost targets (a ghost formation) and the LIDAR will have a difficult time determining which are real and which are ghost targets. There are many other variations of this ghost target technique that are used in both LIDAR and microwave radar countermeasures; however, they use all use the same basic principles. Best regards, your Mad Scientist
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