THE ELECTROMAGNETIC SPECTRUM

Lasers by Hal Hellman, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE ELECTROMAGNETIC SPECTRUM

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).

You would have to count pretty quickly to do this for light, however. Light waves vibrate or oscillate at the rate of some 400 million million times a second. That’s the vibration rate of waves of red light; violet results from vibrations that are just about twice that fast.

With frequencies of this magnitude, discussion and handling of data and dimensions are cumbersome and rather awkward. Fortunately there is another approach. Let’s look again at our ocean waves. We see that there is a regularity about them (before they begin to break on the shore). The distance from one crest to the next is significant and is called the wavelength. Water waves are measured in feet, and in comparable units light waves are recorded in ten-millionths of an inch—again a very cumbersome number. Scientists therefore use the metric system and have standardized a unit called the angstrom, which is equal to the one-hundred-millionth part of a centimeter (10⁻⁸ cm). Thus we find, as shown in Figure 3, that the visible light range runs from the violet at about 4000 angstroms to red at about 7000 angstroms.

Figure 3 The visible light spectrum ranges between approximately 4000 and 7000 angstroms.

At roughly the same time that the wavelength of light was being determined, the German-British astronomer William Herschel performed an interesting experiment. He held a thermometer in turn in the various colors of light that had been spread out by an optical prism. As he moved the thermometer from the violet to the red, the temperature reading rose—and it continued to rise as he moved the instrument beyond the red area, where no prismatic light could be seen.

Thus Herschel discovered infrared rays (the kind of heat we get from the sun) adjoining the visible red light, and at the same time found that they were merely a continuation of the visible spectrum. Shortly thereafter, ultraviolet rays were found on the other end of the visible light band.

One of the most fascinating movements in science has been the constant expansion since then of both ends of the radiating-wave spectrum. The result has come to be called the electromagnetic spectrum, which, as we see in Figure 4, encompasses a wide variety of apparently different kinds of radiation. Above the visible band (the higher frequencies), we find ultraviolet light, X rays, gamma rays, and some cosmic rays; below it are infrared radiation, microwaves, and radio waves. Only a small proportion of the total spectrum is occupied by the visible band. Another point of interest is the inverse relationship between wavelength and frequency. As one goes up the other goes down.

Figure 4 Visible light region spans a tiny portion of the total electromagnetic spectrum.

These many kinds of rays and waves vary tremendously in the ways they interact with matter. But they are all part of a single family. The only difference among them, as with the colors of the rainbow, lies in their wavelengths. In a few cases, as we shall see later, the mode of generation is also different.

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, 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.

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This book is part of the public domain. Hal Hellman (2021). Laser. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/65512/pg65512-images.html

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