Electromagnetic Fields Help (page 3)
The optical telescope was invented long before scientists knew that visible light represents only a tiny part of a continuum of energy wavelengths. Isaac Newton believed that visible light was composed of tiny particles or corpuscles . Today we recognize these particles as photons . However, light is more complex than can be represented by the simple corpuscular theory . The same is true of all forms of radiant energy.
The wave nature of visible light, and of other forms of radiant energy, is the result of synergistic interaction of electrical and magnetic forces. Charged particles, such as electrons and protons, are surrounded by electrical (E) fields . Magnetic poles produce magnetic (M) fields . The fields extend into the space surrounding the charged particles or magnetic poles, and when the fields are strong enough, their effects can be noticed at a considerable distance. When the E and M fields vary in intensity, the result is an electromagnetic (EM) field .
Orderly, well-defined EM fields are generated by voltages or currents that vary in a rhythmic way. Conversely, an EM field can give rise to alternating voltages or currents. These effects can occur over vast distances in space.
Static E And M Fields
If you’ve ever played with permanent magnets, you have noticed the attraction between opposite poles and the repulsion between like poles. Similar effects take place with electrically charged objects. These forces seem to operate only over short distances under laboratory conditions. This is so because static (steady, unchanging) E and M fields weaken rapidly, as the distance between poles increases to less than the smallest intensity we can detect. In theory, the fields extend into space indefinitely.
Physicists have known for a long time that a constant electric current in a wire produces an M field around the wire. The lines of magnetic flux are perpendicular to the direction of the current. It is also known that the existence of a constant voltage difference between two nearby objects produces an E field; the lines of electrical flux are parallel to the direction in which the voltage varies most rapidly with distance. When the intensity of a current or voltage changes with time, things get more interesting.
A fluctuating current in a wire or a variable voltage between two nearby objects gives rise to both an M field and an E field. These fields regenerate each other, so they can travel for long distances with less attenuation than either type of field all by itself. The E and M lines of flux in such a situation are perpendicular to each other everywhere in space. The direction of travel of the attendant EM field is perpendicular to both the E and M lines of flux, as shown in Fig. 18-1.
In order for an EM field to exist, electrons or other charge carriers not only must be moving, but also must be accelerating. That is, their velocity must be constantly changing. The most common method of creating this sort of situation is the introduction of an alternating current (ac) in an electrical conductor. It also can result from the bending of charged-particle beams by E or M fields.
Frequency and Wavelength
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).
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.
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.
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.
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.
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.
The Sun has been observed by radar. Even though the surface is not solid, the outward motion of subatomic particles has been detected: the solar wind . The “surface” of the Sun is different in the radio part of the EM spectrum as compared with the visible-light portion.
The Sun has, of course, no solid surface as does the Earth, Venus, or Mercury. The apparent diameter of the solar globe depends on the EM wavelength at which the Sun is observed. This makes it possible to examine the motion of the gases at different levels. Great turbulence exists there; we know this because Doppler shifts are constantly observed. Radar telescopes allow astronomers to calculate how fast the gases rise and fall as the Sun’s surface boils in an endless storm.
The maximum range of a radar telescope is limited by two factors. First, there is path loss , caused by the sheer physical distances over which radar signals must travel on their way from the antenna to the target and back. Second, the free-space speed of EM-wave propagation is finite. Although 299,792 km/s (186,282 mi/s) seems fast on a terrestrial scale, it is sluggish with respect to the Cosmos.
Path loss increases with distance and mandates more sensitive receivers and more powerful transmitters as the distance gets greater. There is a practical limit to how sensitive any rf receiver can be made. There is also a limit to how much power can be generated in a radar transmitter and a limit to how much gain can be realized with an antenna of manageable size. There is yet room for engineers to design and build larger antennas, more sensitive receivers, and more powerful transmitters. Eventually, however, economic considerations must prevail over scientific curiosity.
The propagation-speed constraint is insurmountable no matter what the size of the hardware budget. An echo from Pluto returns to Earth approximately 10 hours after the signal is sent. An echo from the nearest star system would not return for almost 9 years. Most of the star systems in our galaxy are so far away that the echoes from a radar set will not come back until many human lifetimes have passed. Earth-based radar astronomy will never be useful in the study of objects outside the Solar System.
EM energy at radio frequency has much greater wavelengths than energy in the visible part of the spectrum. The shortest radio microwaves measure approximately 1 mm in length; the reddest visible light has a wavelength of a little less than 0.001 mm. This is a span of a thousandfold, or three mathematical orders of magnitude, and it is called the infrared (IR) spectrum . In terms of frequency, IR lies below red visible light. It is from this fact that IR gets its name: the prefix infra- means “below” or “under.” Our bodies sense infrared as radiant warmth or heat. The IR rays are not literally heat, but they produce heat when they strike an absorptive surface such as human skin.
Stars, galaxies, planets, and other things in the Cosmos radiate at all wavelengths, not only at wavelengths convenient for humans to observe. In some portions of the IR spectrum, the atmosphere of our planet is opaque. Between about 770 nm (the longest visible red wavelength) and 2 micrometers (µm), our atmosphere is reasonably clear, and it is possible to observe IR energy in this wavelength range from surface-based locations. To see celestial images at longer IR wavelengths, the observations must be made from high in the atmosphere or from space.
The Moon, the Sun, and the planets all have been observed in IR, as have some stars and galaxies. IR observing equipment resembles optical apparatus. This is true of telescopes as well as cameras. Similar lenses, films, and sensors are used, and excellent resolution can be obtained. Special kinds of film have been developed recently, making observation possible at longer and longer IR wavelengths.
Stars In Ir
IR astronomy has helped scientists to discover certain peculiar dim stars that seem to radiate most of their energy in the IR range. Visually, such stars appear red and dim. However, like a faintly glowing electric-stove burner, they are powerful sources of IR. These stars have relatively low surface temperatures compared with other stars. At first thought, this seems to be a paradox, but the peak wavelength at which an object radiates is a direct function of the temperature. “Cool” stars produce radiation at predominantly longer wavelengths than “hot” stars. The hottest stars are comparatively weak radiators of IR. When astronomers talk about temperatures of celestial objects, they usually refer to the spectral temperature , which is determined by examining the EM radiation intensity from the object at various wavelengths.
IR astronomy is important in the study of evolving stars and star systems. As a cloud of interstellar dust and gas contracts, it begins to heat up, and its peak radiation wavelength becomes shorter and shorter. A cool, diffuse cloud radiates most of its energy in the radio part of the EM spectrum. Hot stars radiate largely in the UV and x-ray regions. Sometime between the initial contraction of the nebula and the birth of the star, the peak emission wavelength passes through the IR. The observation of IR is also important in the analysis of dying stars. As a white dwarf cools down and becomes a black dwarf, its peak radiation wavelength decreases. On its way toward ultimate cold demise, the star emits, for a certain period of time, most of its energy in the IR.
The characteristic EM radiation from a star is a form of emission known as blackbody radiation . A blackbody is a theoretically perfect absorber and radiator of EM energy at all wavelengths. Any object having a temperature above absolute zero (–273°C or –459°F) has a characteristic pattern of wavelength emissions that depends directly on the temperature. For any EM-radiating object (and this includes everything in the Universe), the emission strength is maximum at a certain defined wavelength and decreases at longer and shorter wavelengths. If EM intensity is graphed as a function of wavelength or frequency, the result is a curve that resembles a statistical distribution (Fig. 18-6).
By observing an object at many different wavelengths including the radio region, the IR, the visible range, and the UV and x-ray spectra, the point of maximum emission can be found. Sometimes it can be inferred even if observations are not actually made at that wavelength by plotting points on an intensity-versus-wavelength graph and connecting the points with a smooth curve. From the maximum-emission wavelength, the temperature of the object can be estimated based on the assumption that it behaves as a blackbody. Figure 18-7 is a rough graph, on a logarithmic scale, of the function employed by scientists for this purpose.
Ultraviolet And Beyond
As the wavelength of an EM disturbance becomes shorter than that of visible violet light, the energy contained in each individual photon increases. The UV range of wavelengths starts at about 390 nm and extends down to approximately 1 nm. The x-ray range extends from roughly 1 nm down to 0.01 nm. (The precise dividing line between the shortest UV and the longest x-ray wavelengths depends on whom you ask.) The gamma-ray spectrum consists of EM waves shorter than 0.01 nm. Cosmic radiation is entirely different in origin; it arises from high-speed subatomic particles thrown off by the most energetic celestial objects.
The Uv Atmospheric “window”
As the wavelength decreases, the atmosphere of Earth becomes highly absorptive at about 290 nm. At still shorter wavelengths, the air is essentially opaque. (This is a good thing because it protects the environment against damaging UV radiation from the Sun.) The atmosphere scatters some EM radiation even in the visible blue and violet parts of the spectrum. This is why the sky appears blue to our eyes. Ground-based observatories can see something of space at wavelengths somewhat shorter than the visible violet, but when the wavelength gets down to 290 nm, nothing more can be seen. At the shortest UV wavelengths ( hard UV and extreme UV ), as in the case of the far IR, it is necessary to place observation apparatus above the atmosphere.
Glass is virtually opaque to UV, so ordinary cameras with glass lenses cannot be used to take conventional photographs in this part of the spectrum. Instead, a pinhole-type device is used, and this severely limits the amount of energy that passes into the detector. While a camera lens has a diameter of several centimeters, a pinhole is less than a millimeter across. This does not present a problem for photographing the Sun or the Moon, but for other celestial objects it is not satisfactory.
For analysis of fainter celestial objects in UV, an instrument called a spectrophotometer is used. This device is a sophisticated extended-range spectrometer in which a diffraction grating (not a glass prism) is used to disperse EM energy into its constituent wavelengths. By moving the sensing device back and forth, any desired wavelength can be singled out for observation, even those in the IR or UV spectrum. The principle of the spectrophotometer is shown in Fig. 18-8. In the long-wavelength or soft-UV range, a photoelectric cell can serve as a sensor. In the hard- and extreme-UV spectra, radiation counters are sometimes used, similar to the apparatus employed for the detection and measurement of x-rays and gamma rays. For photographic purposes, ordinary camera film works in the soft-UV range, but special film, rather like x-ray film, is used to photograph hard-UV and extreme-UV images.
Sources Of Uv
The hot type O and B stars are strong sources of UV radiation. These stars evidently radiate more energy in the UV than in any other part of the EM spectrum. Type O and B stars generally are young stars. Within the visible part of the spectrum, their energy tends to be concentrated at the shortest wavelengths, so such stars look blue to us. The surface temperatures of these stars are much greater than the temperature at the surface of our Sun, which is a type G star. Type O and B stars have surface temperatures ranging from about 15,000 to 25,000°C (27,000 to 45,000°F).
Type A and F stars radiate smaller amounts of UV energy than type O and B stars. Most type A and F stars look white. Still, these stars are hotter than the Sun. Temperatures at their surfaces range from about 8,000 to 15,000°C (14,000 to 27,000°F).
Type K and M stars have the coolest surfaces of any stars, ranging down to about 1,500°C (2,700°F). The greatest amount of visible radiation from such stars falls into the wavelengths corresponding to red and orange. These stars produce comparatively little UV radiation.
Supernovae produce fantastic amounts of visible light, but they also emit large amounts of UV. The explosion of a supernova within a few light-years of the Solar System would be a spectacular sight; the brilliance of the star would exceed that of the full Moon. However, the UV radiation, despite the distance, would compare with that from our own Sun. You could get a “star-burn” from the supernova, even at night, and certainly without realizing it, because the IR intensity would be relatively low. The shortest UV rays, which would cause the most damage to life on our planet, would be largely kept at bay by the atmosphere.
A supernova, after having thrown off a cloud of gas in the process of exploding, causes ionization of the cloud because of UV radiation. This causes the gas to fluoresce, or glow visibly. Such a glowing cloud can be seen from many light-years away. Sometimes nearby stars, of the hot types O and B, cause ionization of interstellar gas. This produces such spectacular astronomical objects as the Great Nebula in Orion, the Horsehead Nebula, and others. Bright emission nebulae betray the presence of UV sources in their vicinity. A fluorescent lightbulb operates on the same principle as the emission nebulae; the coating on the inside of such a bulb is set to glow by UV radiation from the gases within.
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