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University of Birmingham scientists have built an invisibility cloak that makes objects vanish

• Ian Sample, science correspondent
• guardian.co.uk, Tuesday 1 February 2011

It might not work as well as Harry Potter's garment or the potions that transform other unseen heroes in books and on screen, but scientists have built an invisibility cloak that makes everyday objects vanish by bending light to fool the eye.

The cloak – a lump of crystal rather than a flowing cape – can hide only small objects, such as pins and paperclips, but it is the first to work in the range of light visible to humans.

The cloak exploits the natural light-bending properties of calcite, a transparent glass-like crystal, so that an object placed under it is hidden by what appears to be a flat, featureless surface.

The experiments conducted so far pave the way for more sophisticated devices that are capable of hiding much larger objects, the researchers said.

Future cloaks might be used to hide military hardware from view, although the lead researcher, Shuang Zhang at the University of Birmingham, said small cloaks could revolutionise cosmetics by obscuring unsightly blemishes. "If you had a mole on your face, you could potentially cloak it so it won't be seen," Zhang said. "Though you do need a fairly large cloak to hide even a small thing."

The device needs some technical tweaks before it passes muster. Although it can hide small objects from view, the cloak itself – roughly the size of a small paperweight – is visible.

Another shortcoming is that it only works when light is polarised in a particular plane, meaning objects only disappear completely when viewed through a filter.

Progress on invisibility cloaks has taken great leaps since 2006, when a group led by Sir John Pendry at Imperial College London, described a technique called transformation optics. The method allows scientists to control light and other electromagnetic waves, which allows for the design of materials that steer light around objects, so making them disappear from view.

Recent invisibility attempts have centred on man-made composite materials with exquisitely fine structures that bend electromagnetic waves the wrong way.

While these can hide tiny objects from microwaves and infrared light, they only work for specific wavelengths and cannot make objects invisible to humans.

In the latest study, the Birmingham group joined forces with Pendry and scientists at the Technical University of Denmark to build the first cloaking device that works in visible wavelengths of light. They found that calcite, a naturally-occurring crystal, had just the right properties. The cloak uses two calcite prisms joined together to make a pyramid with a slight recess in the base. The underside of the pyramid is then coated with gold to make it reflective.

The cloak hides objects placed underneath it because light rays passing through it are bent, making the base of the pyramid look flat. "The cloaked region is the space at the bottom of the calcite prism," Zhang said. "Anything you put in there won't be seen from outside.

"If you put a pin or a paperclip in there you see nothing. From the outside, you just see a flat surface."

The research is published in the journal Nature Communications. A major drawback is that the cloak, though transparent, is not invisible. Zhang said it may be possible to coat the cloak and make it less visible. Tests at his laboratory show it is almost completely invisible in water.

Larger calcite crystals are needed to build cloaks that make bigger objects invisible, Zhang said. One crystal, which is seven metres long and two metres high, could hide an object the size of a large dog.

"Another limitation is that our cloak has to be placed on a surface to work," Zhang said. "Harry Potter's cloak makes things invisible that are in free space - and that is much harder to do."

Hidden history

The dream of invisibility has enthralled since antiquity, when Perseus donned a cap to slay Medusa, and Plato pondered the pitfalls of the mythic Ring of Gyges, which rendered its wearer invisible: would an invisible man who lived with no fear of consequences behave morally?

More recently, the Romulans "cloaked" to stalk Captain Kirk, James Bond's Aston Martin Vanquish vanished, and Harry Potter walked unseen through the corridors of Hogwarts beneath an enchanted gown.

In fiction, invisibility is the stuff of caps, rings, cloaks and mysterious rays, but in some cases, fact is not so far away. The invisible woman vanished from sight by bending light around herself, a concept used by scientists building cloaks today. An onlooker will see nothing if light bends around an object, instead of bouncing off it.

In Die Another Day, Pierce Brosnan's invisible car was made so when the car cloaked itself in a picture of the background, as filmed by cameras on the bodywork. This month, engineers outlined similar plans to make British army tanks melt into the background, by covering them with Kindle-like screens that show snapshots of the surrounding scenery.

In 2003, Susumu Tachi at Tokyo University demonstrated an invisibility jacket that projected an image of the wearer's background onto the front of the coat.

The half-hearted attempt by Predator, in the film of that name, is perhaps fiction's most accurate portrayal of invisibility cloaks. The object is still visible, if only vaguely, as light is always reflected somewhere.

Most cloaking devices in labs today use fine-structured "metamaterials" to bend light. They work only for tiny objects and certain kinds of electromagnetic waves, such as infra-red light or microwaves. Building a cloak that hides large objects from visible light remains a serious challenge.

bend: wyginać, zginać

blemish: wyprysk (na twarzy)

bounce off: odbijać się

calcite: kalcyt

capable of: zdolny do

cape: peleryna

coated: pokryty warstwą

composite: złożony

don: przywdziać

enchanted: zaczarowany

enthral: fascynować

exploit: wykorzystywać

exquisitely: znakomicie

featureless: pozbawiony właściwości

garment: strój

infrared: podczerwień

lump: bryła, grudka

obscure: przesłaniać

onlooker: widz

outline: szkicować

paperweight: przycisk do papieru

pass the muster: zdać test

pave the way for: utorować (czemuś) drogę

pitfall: pułapka, zagrożenie

ponder: rozmyślać, zastanawiać się

prism: graniastosłup

recess: nisza

render: sprawiać, czynić

slay: zabić

sophisticated: wyszukany, skomplikowany

tweak: ulepszenie

unsightly: szkaradny

vaguely: niejasno, ogólnikowo

vanish: znikać

wavelength: długość fali

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