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Invisibility lens: Science of bending light around objects

Posted: 02 Oct 2014     Print Version  Bookmark and Share

Keywords:cloak  invisility  light  lens 

Harry Potter's invisibility cloak has inspired a team of scientists from the University of Rochester to develop a cloaking device to hide objects from a range of angles using inexpensive, readily available materials.

"There've been many high tech approaches to cloaking and the basic idea behind these is to take light and have it pass around something as if it isn't there, often using high-tech or exotic materials," said John Howell, a professor of physics at the University of Rochester.

Forgoing the specialised components, Howell and graduate student Joseph Choi developed a combination of four standard lenses that keeps the object hidden as the viewer moves up to several degrees away from the optimal viewing position.

Joseph Choi

Doctoral student Joseph Choi is pictured with a multidirectional `perfect paraxial? cloak using 4 lenses. Source: University of Rochester

"This is the first device that we know of that can do three-dimensional, continuously multi-directional cloaking, which works for transmitting rays in the visible spectrum," said Choi, a PhD student at Rochester's Institute of Optics.

Many cloaking designs work fine when you look at an object straight on, but if you move your viewpoint even a little, the object becomes visible, explains Howell. Choi added that previous cloaking devices could also cause the background to shift drastically, making it obvious that the cloaking device is present.

In order to both cloak an object and leave the background undisturbed, the researchers determined the lens type and power needed, as well as the precise distance to separate the four lenses. To test their device, they placed the cloaked object in front of a grid background. As they looked through the lenses and changed their viewing angle by moving from side to side, the grid shifted accordingly as if the cloaking device was not there. There was no discontinuity in the grid lines behind the cloaked object, compared to the background, and the grid sizes (magnification) matched.

The Rochester Cloak can be scaled up as large as the size of the lenses, allowing fairly large objects to be cloaked. And, unlike some other devices, it's broadband so it works for the whole visible spectrum of light, rather than only for specific frequencies.

Their simple configuration improves on other cloaking devices, but it's not perfect. "This cloak bends light and sends it through the centre of the device, so the on-axis region cannot be blocked or cloaked," said Choi. This means that the cloaked region is shaped like a doughnut. He added that they have slightly more complicated designs that solve the problem. Also, the cloak has edge effects, but these can be reduced when sufficiently large lenses are used.


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