Figure 4 Visible light region spans a tiny portion of the total electromagnetic spectrum.
| Frequency (cps) | Wavelength | |
|---|---|---|
| Angstroms | ||
| Cosmic rays | ||
| 10²² | 0.0001 | |
| 0.001 | ||
| 10²⁰ | Gamma rays | 0.01 |
| 0.1 | ||
| 10¹⁸ | X rays | 1 |
| 10 | ||
| 10¹⁶ | Ultraviolet radiation | 100 |
| 1,000 | ||
| Visible light | ||
| 10¹⁴ | 10,000 | |
| Infrared radiation | 100,000 | |
| Angstroms | ||
| 0.01 | ||
| 10¹² | Millimeter waves | 0.1 |
| 10¹⁰ | Microwaves, radar | 1 |
| 10 | ||
| 10⁸ | TV and FM radio | 100 |
| Short wave | 1,000 | |
| 10⁶ | AM radio | 10,000 |
| Low frequency communications | 100,000 | |
| 10,000 = 10⁴ | 1,000,000 |
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[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.
Figure 5 (a) Temporally coherent radiation. (b) Temporally incoherent radiation.