In fantasy movies, people wear a cloak and disappear. In 2006, scientists really made an invisibility cloak. It bent waves around a small copper object, so the object was hard to find.

But it worked only with microwaves, not with the light we see. A cloak that hides a person from our eyes is still very hard to make. Also, the person inside could not see anything!

We see an object because light bounces off it or is blocked by it and then reaches our eyes. If light could flow smoothly around an object and continue on its original path, the object would seem to vanish, just as a stone in a river disappears from view downstream if the water flows perfectly around it. In 2006, scientists used a metamaterial, an artificial material built from tiny repeated structures, to guide microwaves around a copper cylinder and make it hard to detect.

However, a cloak for visible light is far more difficult. It would need to work for every color and from every angle, and its structures would have to be extremely small. And the person inside would not be able to see out.

The invisibility cloak, long a fixture of fantasy fiction, was demonstrated in a laboratory in 2006, though with an important catch. That year, John Pendry and David Smith's group published a theory in Science describing how electromagnetic waves could be steered around an object, and Schurig and colleagues used it to build a metamaterial device that concealed a copper cylinder at microwave frequencies. Metamaterials are engineered structures, such as tiny metal rings arranged in regular patterns, whose features are smaller than the wavelength they control, allowing them to bend waves in ways natural materials cannot.

Later experiments using calcite crystals hid small objects from visible light under restricted conditions. A cloak that hides a person from human eyes, however, faces fundamental obstacles. Metamaterials typically work over a narrow range of wavelengths, whereas white light contains every color; many designs fail when viewed from the side; and structures smaller than visible wavelengths would have to be fabricated across an area large enough to cover a body. Theoretical work in 2016 showed that passive cloaks face a trade-off between bandwidth and degree of concealment. Ironically, a perfect cloak would also leave its wearer in darkness.