The New York Police Department is working with the Department of Defense to develop a scanning device that would allow them to detect concealed firearms on a person 80 feet away. The scanner detects electromagnetic waves with a frequency in the terahertz range. Terahertz waves sit at the higher frequency end of infrared on the electromagnetic spectrum, just before the microwave range. The device works the same way an infrared detector does. Just as our bodies emit infrared radiation, so do they emit terahertz waves. These waves can pass through non-conducting material such as paper or clothing but are blocked by conducting material such as a piece of metal – or a gun. So if a person has a piece of metal under their jacket, such as a gun, the police will see the telltale outline of the weapon. The waves also pass unperturbed through wood and brick so the device can scan through walls.