The first three-dimensional visible light invisibility cloak
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Unlike other cloaking devices we’ve covered in the past, Rochester’s invisibility cloak is fashioned out of good old lenses — four of them, to be precise. When the lenses are arranged correctly (as dictated by their focal lengths), they create a region of invisibility. The caveat, of course, is that the observer needs to be looking through the lenses (up to an angle of 15 degrees off-center) — but, you can make the lenses as large as you like, so you can still do a pretty good job of making things disappear. Because these are just your standard lenses — like you might find in your spectacles or DSLR — they work across the entire visible light spectrum, and also a few other frequencies as well. The grid lines behind the object show that the very little distortion is introduced by the lenses, meaning the quality of invisibility is quite high.
Rochester’s invisibility cloak is very simple. Basically, the two outer lenses focus the light from a wide area onto two smaller lenses in the middle. As you can see in the diagram below, this setup creates a region where incident light can’t reach the object, and reflected light can’t reach the observer. Somewhat oddly, the region of invisibility is shaped like a hollow cylinder — or a very tall doughnut, if you prefer. Again, if you make the lenses big enough, the cloaked region would be big enough to hide most things — but on a smaller scale, there are not many doughnut-shaped objects that might benefit from invisibility. Except if your dad/dog/kid keeps eating your last doughnut, of course. [Research paper: arXiv:1409.4705 - "Paraxial Ray Optics Cloaking"]
While Rochester’s four-lens method won’t be allowing for Harry Potter-style invisibility cloaks any time soon – we still need some kind of wonder or metamaterial for that — there are still some interesting applications. With some particularly fine lenses — perhaps fresnel lenses to reduce the size of the setup — surgeons might be able to use this setup to see through their hands, or car/truck drivers might be able to see through the front and rear pillars of their vehicle.
Funnily enough, the Rochester researcher behind this study — John Howell — is the same guy who last year created a human-scale invisibility cloak for $150 — with his kids. While that “cloak” was made out of mirrors instead of lenses, both solutions have one thing in common: They can both be built from cheap, off-the-shelf components. So, go forth and build your own invisibility cloak! And upload David Copperfield-like illusions to YouTube!
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