Electromagnetic waves in the terahertz range, known as T-rays, are used in highly sensitive security scanners, some prototype medical devices and a host of other technologies. Like X-ray scanners, T-ray scanners can penetrate where optical light cannot — through paper, clothing, and you. They can also sense any molecule, because every one has a unique signature in the THz range, according to researchers at Imperial College London. This makes them useful as drug sniffers or medical imagers — they could sense molecules associated with cancer or other diseases, for instance. But THz imaging devices, like quantum cascade lasers, require huge amounts of energy and must operate at low temperatures, so they're expensive and therefore not very common.