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Electromagnetic Fields Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Frequency and Wavelength

Frequency

The frequency of an EM field can be very low, such as a few cycles per second, or hertz (abbreviated Hz), and can range upward into the thousands, millions, billions, or trillions of hertz. A frequency of 1,000 Hz is a kilohertz (kHz); a frequency of 1,000 kHz is a megahertz (MHz); a frequency of 1,000 MHz is a gigahertz (GHz); and a frequency of 1,000 GHz is a terahertz (THz).

Wavelength

Electromagnetic waves travel through space at the speed of light, which is approximately 2.99792 × 10 8 m/s. This is often rounded up to 3.00 × 10 8 m/s, expressed to three significant figures. The wavelength of an EM field in free space gets shorter as the frequency becomes higher. At 1 kHz, the wavelength is about 300 km. At 1 MHz, the wavelength is about 300 m. At 1 GHz, the wavelength is about 0.300 m or 300 mm. At 1 THz, an EM signal has a wavelength of 0.3 mm—so small that you would need a magnifying glass to see the waves, if they were in fact directly visible.

The frequency of an EM wave can get much higher than 1 THz; some of the most energetic rays known have wavelengths of 0.000001 nanometer (10 –6 nm). The nanometer equivalent to 10 –9 m and is used commonly by scientists to measure the wavelengths of EM disturbances at visible wavelengths and shorter. A microscope of great magnifying power would be needed to see an object with a length of 1 nm. Less commonly, the Ångström (Å) is used to denote extremely short wavelengths. This unit is 1/10 of a nanometer; that is, 1 Å = 10 –10 m = 0.1 nm.

Many Forms

The discovery of EM fields led to the “wireless” radio and ultimately to the sophisticated and complex variety of 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 come the microwaves . Then comes infrared (IR), or “heat rays.” After that comes visible light, ultraviolet (UV) radiation, x-ray energy, and gamma-ray energy.

In the opposite, and less commonly imagined, sense, EM fields can exist at frequencies far below those of radio signals. Some extremely-low-frequency (ELF) fields have frequencies less than the 50 or 60 Hz of ac utility electricity. 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 Spectrum

The wavelengths of the lowest-frequency EM fields can extend, at least theoretically, for light-years. The shortest gamma rays have, as we have already mentioned, wavelengths that measure only a tiny fraction of a nanometer. In between these extremes lie all the forms of EM energy.

The Em Wavelength Scale

To illustrate the range of EM wavelengths, scientists often use a logarithmic scale. It is necessary to use a logarithmic scale 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 and shows wavelengths from 10 8 down to 10 –12 m. Each division, in the direction of shorter wavelength, represents a tenfold decrease, known as a mathematical order of magnitude (not to be confused with star magnitude). Utility ac is near the top of this scale; the wavelength of 60-Hz ac in free space is quite long. Gamma rays are at the bottom; their EM wavelengths are tiny.

Observing the Invisible Electromagnetic Fields The Em Wavelength Scale

Figure 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.

From this example, it is easy to see that visible light takes up only a tiny sliver of the EM spectrum. However, this diagram makes the visible portion of the realm look much larger than it is in linear proportion. If the scale were linear in this illustration, the visible slice would be thinner than the diameter of an atom. Along the right-hand scale, visible wavelengths are denoted in nanometers.

How Little We See!

The next time you get a chance, look 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, blue appears the same as black, and crimson appears the same as gray or white. 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, our view of the world would be much different; we would essentially be 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 wavelength appears red to our eyes, and energy at the shortest visible wavelength appears violet. The intervening wavelengths show up as orange, yellow, green, blue, and indigo.

In order to “see” EM energy waves above and below the visible spectrum, we need special apparatus. Astronomers have devised an amazing variety of instruments that can detect energy all the way from the radio spectrum through microwaves, IR, visible, UV, x-rays, and even gamma rays.

Atmospheric Em Transparency

The observation of the Universe at the visible wavelengths is not greatly hindered by the atmosphere of Earth. Some diffusion and refraction take place, but the air is essentially transparent at the range of 770 to 390 nm. At some other wavelengths, however, energy from outer space can’t make it to the ground, so we can’t detect it unless we put instruments high up in the atmosphere or in outer space.

At radio wavelengths greater than a few meters, Earth’s ionosphere , at altitudes ranging from about 60 km (40 mi) to 400 km (250 mi), causes absorption, refraction, and reflection of EM waves. This keeps signals from space away from us at certain radio frequencies. This same effect keeps shortwave radio waves near the Earth and makes long-distance communication possible.

At UV wavelengths, the ozone in the air is highly absorptive. In the x-ray and gamma-ray parts of the EM spectrum, our atmosphere completely obscures our view of the heavens. However, some portions of the invisible spectrum are observable with ground-based equipment, especially the wavelengths between microwaves and visible light.

One of the most significant developments in the exploration of space at invisible wavelengths has been the radio telescope. Radio astronomy has become a refined science since the development of wireless communications equipment and in particular since the end of World War II in the 1940s. High-altitude rockets, orbiting satellites, and the advent of the space age have opened up the realms of UV, x-ray, and gamma-ray astronomy.

Radar Astronomy—The Moon and Venus

The huge parabolic antennas used in some radio telescopes, as well as other forms of high-gain antenna systems, can work for transmitting in the same way as they function for receiving. The large power gain developed in the interception of a faint signal from space also can increase the effective transmitted power of a signal greatly. By sending out short pulses of radio energy, generated by a transmitter in the laboratory, and then listening for possible echoes, astronomers can make accurate determinations of the distances to other objects in the Solar System. The use of radio telescopes for distance determination, motion analysis, and surface mapping of extraterrestrial objects is called radar astronomy .

The Moon And Venus

The distance to Earth’s Moon has been refined to within a few centimeters, and it is possible to tell how fast the Moon is moving toward or away from Earth as it revolves around our planet. The surface of the Moon has been mapped by radar, and the quality of detail rivals that of optical photographs. Radar astronomy has proven to be a valuable tool in the study of the surface of Venus because that planet’s thick clouds make it impossible to see the surface with optical apparatus.

The main challenge to pursuing radar investigations of the planets has been the development of radar sets with enough power and sensitivity. The planets are very distant in terms of human-made radio transmissions. In addition, they do not reflect radio signals the way a mirror reflects visible light. Only a minuscule part of the rf signal reaching a planet is reflected back in the general direction of the Earth; of this reflected wave, only a tiny portion ever gets back to the antenna. Radar technology eventually was perfected to overcome the path loss between Earth and Venus. The radar telescope enabled scientists to discover that Venus has a retrograde motion on its axis. Before that time, astronomers had made educated guesses concerning the planet’s rotation based on the motion of the cloud tops, and these guesses had all been wrong.

Radar mapping of the surface of Venus was attempted because rf signals easily penetrate the visually opaque clouds. Astronomers found that the surface is solid but irregular. High-resolution maps were obtained when the planet was scrutinized using radar sets aboard Venus-orbiting spacecraft. The solidity of the surface was not known with certainty until it was verified by radar.

Mercury

Observations of Mercury have been made by radar. Surface details of Mercury are difficult to resolve with optical telescopes based on the Earth’s surface because that planet is always near the Sun in the sky. The rotation rate of Mercury was once believed to be 88 Earth days, identical with its period of revolution around the Sun. The radar telescope revealed that one rotation is completed in 59 Earth days. Mercury has a day of its own, but it is long and strange by our standards.

Echoes from the planet Mercury have been used to verify one of the predictions of Albert Einstein’s general theory of relativity. According to Einstein’s equations, radio waves passing close to a massive object, such as the Sun, should appear to “slow down” because of the curvature of space in the gravitational field. All radiant energy, according to general relativity, is affected in this way near massive celestial objects.

Experimenters bounced radar signals off Mercury as it passed on the far side of the Sun (Fig. 18-5). According to general relativity, an illusion should occur in which Mercury seems to deviate about 65 km outside its orbital path as it passes behind the Sun. This deviation was found, and it takes place to the exact extent predicted by Einstein. The echoes are slightly delayed as the signals pass near the Sun on their way to and from Mercury.

Observing the Invisible Radar Astronomy Mercury

Figure 18-5. Mercury appears to deviate outside its orbit when it passes behind the Sun. (The extent of the deviation is exaggerated in this drawing.)

Meteor Orbits

Radar astronomy is useful in the study of meteors. Meteors apparently come, for the most part, from inside the Solar System; they are not interstellar wanderers. This can be deduced by determining the velocities of large numbers of meteors relative to Earth. The velocities of meteors as they enter the atmosphere can be measured accurately using radar. Such measurements are difficult or impossible to do by visual means. From the radar information, an astronomer can figure out the original paths of meteors through space. These paths always turn out to be orbits around the Sun.

Meteors arrive during the day as well as at night, and the radar telescope can “see” them in visual daylight as well as in visual darkness. This gives the radar-equipped observer another advantage over the visual observer.

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