Infrared and Millimeter Waves………..
Unguided infrared and millimeter waves are widely used for short-range communication. The remote controlsused on televisions, VCRs, and stereos all use infrared communication. They are relatively directional, cheap,and easy to build but have a major drawback: they do not pass through solid objects (try standing between yourremote control and your television and see if it still works). In general, as we go from long-wave radio towardvisible light, the waves behave more and more like light and less and less like radio.
On the other hand, the fact that infrared waves do not pass through solid walls well is also a plus. It means thatan infrared system in one room of a building will not interfere with a similar system in adjacent rooms orbuildings: you cannot control your neighbor’s television with your remote control. Furthermore, security ofinfrared systems against eavesdropping is better than that of radio systems precisely for this reason. Therefore,no government license is needed to operate an infrared system, in contrast to radio systems, which must belicensed outside the ISM bands. Infrared communication has a limited use on the desktop, for example,connecting notebook computers and printers, but it is not a major player in the communication game.