The band of radiation stretching from the infrared to cosmic rays has been, up to now, largely the concern of physical scientists. Because of their high frequencies, these radiations are generally handled, when calculations or measurements must be made, in terms of wavelength. Radio and microwaves[5], on the other hand, have been more in the domain of communications engineers and are more likely to be discussed in terms of frequency. Thus it is that your radio is marked off in kilocycles, or thousands of cycles per second, while light is described as radiation in the 4000 to 7000 angstrom band. The relative newness of the various radiations has kept scientists busy learning about them and, as information and experience have accumulated, putting them to work. RADIO WAVES One of the first of the newly discovered electromagnetic radiations to be put to work was the radio wave, which is characterized by long wavelength and low frequency.[6] The low frequency makes it relatively easy to produce a wave having virtually all its power concentrated at one frequency. The advantage of this capability becomes obvious after a moment’s thought. Think for example of a group of people lost in a forest. If they hear sounds of a search party off in the distance, all likely will begin to shout in various ways for help. Not a very efficient process, is it? But suppose all the energy going into the production of this noise could be concentrated in a single shout or whistle. Clearly, their chances of being found would be much improved. [Illustration: Figure 5 _(a) Temporally coherent radiation. (b) Temporally incoherent radiation._] The single frequency capability of radio waves has been given the name _temporal coherence_ (or coherence in time) and is illustrated in Figure 5. Part _a_ shows a single sine wave, the common way of representing electromagnetic radiation, and particularly _temporally coherent radiation_. In _b_ we see what _temporally incoherent radiation_ (such as the mixed sounds of the stranded party) would look like. It was on Christmas Eve 1906 that music and speech came out of a radio receiver for the first time. Today the sight of someone walking, riding, or studying with an earpiece plugged into a transistor radio is common. But the early radio enthusiasts _had_ to wear earphones because it takes considerable power to activate a loudspeaker and the received signal was quite weak. Some means of increasing, or amplifying, the signal was needed if the process was to advance beyond this primitive stage.[7]