Still another important field of application is that of communications. Perhaps because it is less spectacular than burning holes in razor blades, we haven’t heard as much about it. Yet there are probably more physicists and engineers working on adapting the laser for use in communications than on any other single laser project. The reason for this is the fact that existing communications facilities are becoming overloaded. Space on transoceanic telephone lines is already at a premium, with waiting periods sometimes running into hours. Radio “ham” operators have been threatened with loss of some of their best operating frequencies to meet the demand of emerging nations of Africa for new channels. Television programs must compete for space on cross-country networks with telephone, telegraph, and transmission of data. The increasing use of computers in science, business, and industry will strain our facilities still further. Communication satellites will help, but they will not give us the whole answer; and much development work remains to be done on satellites. [Illustration: Figure 2 _Precision control of a machine tool by laser light._] Why the interest in the laser for communications? In a recent experiment all seven of the New York TV channels were transmitted over a single laser beam. In terms of telephone conversations, one laser system could theoretically carry 800,000,000 conversations—four for each person in the United States. In this booklet we shall learn what there is about the laser that gives it so much promise. We shall investigate what it is, how it works, and the different kinds of lasers there are. We begin by discussing some of the more familiar kinds of radiation, such as radio and microwaves, light and X rays. THE ELECTROMAGNETIC SPECTRUM Some 85% of what man learns comes to him through his vision in response to the medium of light. Yet, ironically, it wasn’t until the end of the 17th century that he first began to get an inkling of what light really is. It took the great scientific genius Isaac Newton to show that so-called white light is really a combination of all the colors of the rainbow. A few years later the Dutch astronomer Christiaan Huygens introduced the idea that light is a wave motion, a concept finally validated in 1803 when the British physician Thomas Young ingeniously demonstrated interference effects in waves. Thus it was finally realized that the only difference between the various colors of light was one of wavelength. For light was indeed found to be a wave phenomenon, no different in principle from the water waves you have seen a thousand times. If you stand at the seashore, you can easily count the number of waves that approach the shore in a minute. Divide that number by 60 and you have the frequency of the wave motion in the familiar unit, cycles-per-second (cps).[1]