Out of mind - out of sight [metamaterials]

The idea behind invisibility is to make light travel around an object in the same time that it would take it to go in a straight line if the object was not there. To achieve this, the object must be surrounded by a material that can propagate light at a speed faster than in a vacuum. Metamaterials, where the structure as well as the chemistry influences the optical properties, have been known for a long time. Metamaterials with a negative refractive index already been produced by bending the light in the opposite way from the expected one - the sort of approach required to make sure that something cannot be seen. Metamaterials work by resonance at a given frequency (related to the wavelength) and will only resonate perfectly at that very frequency. Because the range for visible light is so broad, people predict that current approaches would only make an object invisible at a particular colour.