A fish swims into an enclosure and disappears, while the pondweed behind remains perfectly visible. A cat climbs into a glass box and vanishes, and again the scene behind the box remains visible through the glass. This latest addition to the science of invisibility cloaks is one of the simplest implementations so far, but there’s no denying its striking impact.
The ‘box of invisibility’ has been designed by a team of researchers at Zhejiang University in Hangzhou, China, led by Hongsheng Chen, and their coworkers. The box is basically a set of prisms made from high-quality optical glass that bend light around any object in the enclosure around which the prisms are arrayed, the researchers describe in a paper posted on the online repository arXiv.
Chen and his coworker Bin Zheng first unveiled the principle last year with a six wedge-shaped prisms arranged to form a hexagon that could hide small objects. The researchers now used a similar but larger hexagon of prisms placed in a fish tank. As a fish swims through the central hole, it disappears while the pondweed behind the cloak remains perfectly visible.
A second demonstration uses a square arrangement of eight prisms with a central cavity large enough for a cat to climb inside. The researchers project a movie through the cloak and onto a screen behind it. Seen from the front, parts of the cat vanish as it sits in the cavity or pokes its head inside, while the scene behind — a field of flowers, with a butterfly flitting between them — can be seen through the glass (see figure below and video at bottom).
As well as being visible themselves, these cloaks only work for certain viewing directions. A sophisticated observer would also be able to measure that different rays of light have traveled different distances inside the device.
As such, the trick is arguably closer to ‘disappearances’ staged in Victorian music hall using arrangements of slanted mirrors than to the modern use of substances called metamaterials to achieve invisibility by guiding light rays in unnatural ways. These are materials made from arrays of electrically conducting components that interact with light so as to create exotic optical effects such as negative refractive index.