EM Fields Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 12, 2011

Many Forms

The discovery of EM fields led ultimately to the variety of wireless communications systems we know today. Radio waves are not the only form of EM radiation. As the frequency increases above that of conventional radio, we encounter new forms. First comes microwaves . Then comes infrared (IR) or “heat rays.” After that comes visible light, ultraviolet (UV) radiation, x-rays, and gamma (γ) rays.

In the opposite, and less commonly imagined, sense, EM fields can exist at frequencies far below those of radio signals. In theory, an EM wave can go through one complete cycle every hour, day, year, thousand years, or million years. Some astronomers suspect that stars and galaxies generate EM fields with periods of years, centuries, or millennia.

The Em Wavelength Scale

To illustrate the range of EM wavelengths, we use a logarithmic scale. The logarithmic scale is needed because the range is so great that a linear scale is impractical. The left-hand portion of Fig. 18-2 is such a logarithmic scale that shows wavelengths from 10 8 m down to 10 −12 m. Each division, in the direction of shorter wavelength, represents a 100-fold decrease, or two orders of magnitude. Utility ac is near the top of this scale; the wavelength of 60-Hz ac in free space is quite long. The gamma rays are denoted approximately at the bottom; their EM wavelengths are tiny. It is apparent here that visible light takes up only a tiny sliver of the EM spectrum . In the right-hand scale, visible wavelengths are denoted in nanometers (nm).

Forms of Radiation EM Fields How Little We See!

Fig. 18-2 . The EM spectrum from wavelengths of 10 8 m down to 10 −12 m, and an exploded view of the visible-light spectrum within.

How Little We See!

To get some idea of what a small EM “window” is represented by the visible-light wavelengths, try looking through a red- or blue-colored piece of glass or cellophane. Such a color filter greatly restricts the view you get of the world because only a narrow range of visible wavelengths can pass through it. Different colors cannot be ascertained through the filter. For example, when a scene is viewed through a red filter, everything is a shade of red or nearly red. Blue appears the same as black, bright red appears the same as white, and maroon red appears the same as gray. Other colors look red with varying degrees of saturation, but there is little or no variation in the hue. If our eyes had built-in red color filters, we would be pretty much color-blind.

When considered with respect to the entire EM spectrum, all optical instruments suffer from the same sort of handicap we would have if the lenses in our eyeballs were tinted red. The range of wavelengths we can detect with our eyes is approximately 770 nm at the longest and 390 nm at the shortest. Energy at the longest visible wavelengths appears red to our eyes, and energy at the shortest visible wavelengths appears violet. The intervening wavelengths show up as orange, yellow, green, blue, and indigo.

EM Fields Practice Problems

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