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Physicists have figured out the optical parameters for a magic trick they characterize as a kind of "invisibility cloak" — and unlike most magicians, they're only too willing to show you how it's done.
Invisibility cloaks have been getting a lot of press over the years. That's not only because Harry Potter put his fictional cloak to such good use, but also because researchers have been using high-tech metamaterials to create structures capable of bending light around an object to keep it hidden.
The invisibility device developed by researchers at the University of Rochester bends light as well, but not in the ways that magical cloaks or metamaterials do.
"We just figured a very simple way of doing that can just be using standard lenses, and things that we normally find in the lab," physics professor John Howell said in a video explaining the setup.
Howell enlisted two of his sons for a similar invisibility demonstration last year. This time around, Howell and graduate student Joseph Choi are co-authors of a paper about the "Rochester Cloak" that has been submitted to the journal Optics Express.
"This is the first device that we know of that can do three-dimensional, continuously multidirectional cloaking, which works for transmitting rays in the visible spectrum," Choi said in a University of Rochester news release.
In its simplest form, the device is an arrangement of four lenses, placed in such a way that when you look through them, you clearly see what's beyond the farthest lens — but not necessarily what's placed between the lenses. You can stick a ruler, or your finger, or a secret decoder ring between the lenses, and nothing will show up.
Yes, the effect is a bit reminiscent of a sideshow magic trick, and there are some drawbacks: For example, the simplest setup involves bending light rays through the center of the lenses, which means the zone of invisibility is actually a doughnut with a hole in the middle. If an object blocks the central axis, the invisibility effect is spoiled. (You'll notice the objects that disappear in the video are all slightly off-center.) Slightly more complicated designs get rid of that hole of visibility.
Despite the limitations of the Rochester Cloak, Howell thought of several potential applications. For example, optical arrangements could be set up in an operating room to let a surgeon "look through his hands to see what he is actually operating on," he said. You could also create a system to help truck drivers see through the obstructions on their vehicles (although a plain old rear-view camera would also work).
Perhaps the best application is to build the Rochester Cloak in physics labs, as a teaching tool to show students how math and science can do magic. So how do you make a Rochester Cloak? Here's a diagram and the instructions from the University of Rochester.
1. Purchase two sets of two lenses with different focal lengths: two with focal length f1 (say, 200mm) and two with focal length f2 (say, 75mm).
2. Separate the first two lenses by the sum of their focal lengths (t1=f1+f2). In the example, 200mm + 75mm = 275mm, or 10.8 inches.
3. Repeat Step 2 for the other two lenses.
4. Separate the two sets by the distance t2 = 2f2 (f1+f2) / (f1-f2). In the example, that would be 330mm, or 13 inches.
- Achromatic lenses provide best image quality. The researchers used 50mm achromatic doublets.
- Fresnel lenses can be used to reduce the total length.
- Smaller total length should reduce edge effects and increase the range of angles for "invisibility."
- For an easier but less ideal cloak, you can try the three-lens cloak described in the paper.