Invisible Cloak Of Harry Potter Not Only A Dream

   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."