You have no doubt seen mirages on the distant surfaces of hot highways before, looking like pools of water shimmering on the asphalt. Such illusions are caused by hot air above the road, which refracts light waves coming down into it from the cooler air above - in other words, the supposed "water" is actually the sky, its image being bent toward you by the low-lying hot air. Well, scientists at the University of Texas at Dallas have put the same principle to work in the lab, and created an invisibility cloak that can be easily switched on and off.
The cloak itself is composed of a thin transparent layer of highly-aligned carbon nanotubes, which are known for their ability to conduct and disperse heat. This sheet is drawn taut on a mounting apparatus, and then placed in front of the object that needs to be hidden.
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When viewed through the clear sheet, the object is completely visible. When an alternating electric current or a pulse of electromagnetic radiation is ran through the sheet, however, it becomes hot, and also spreads that heat to the air immediately around it. This causes a steep temperature gradient, which in turn causes light rays reflecting off the object to bend, instead of proceeding straight to the viewer's eye. As a result, the object seems to disappear.
As soon as the current is turned off, the effect ceases. The process is said to work particularly well underwater, as the video below illustrates.