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Electromagnetic Spectrum

ELECTROMAGNETIC SPECTRUM

CONCEPT

One of the most amazing aspects of physics is the electromagnetic spectrumradio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma raysas well as the relationship between the spectrum and electromagnetic force. The applications of the electromagnetic spectrum in daily life begin the moment a person wakes up in the morning and "sees the light." Yet visible light, the only familiar part of the spectrum prior to the eighteenth and nineteenth centuries, is also its narrowest region. Since the beginning of the twentieth century, uses for other bands in the electromagnetic spectrum have proliferated. At the low-frequency end are radio, short-wave radio, and television signals, as well as the microwaves used in cooking. Higher-frequency waves, all of which can be generally described as light, provide the means for looking deep into the universeand deep into the human body.

HOW IT WORKS

Electromagnetism

The ancient Romans observed that a brushed comb would attract particles, a phenomenon now known as static electricity and studied within the realm of electrostatics in physics. Yet, the Roman understanding of electricity did not extend any further, and as progress was made in the science of physicsafter a period of more than a thousand years, during which scientific learning in Europe progressed very slowlyit developed in areas that had nothing to do with the strange force observed by the Romans.

The fathers of physics as a serious science, Galileo Galilei (1564-1642) and Sir Isaac Newton (1642-1727), were concerned with gravitation, which Newton identified as a fundamental force in the universe. For nearly two centuries, physicists continued to believe that there was only one type of force. Yet, as scientists became increasingly aware of molecules and atoms, anomalies began to arisein particular, the fact that gravitation alone could not account for the strong forces holding atoms and molecules together to form matter.

FOUNDATIONS OF ELECTROMAGNETIC THEORY.

At the same time, a number of thinkers conducted experiments concerning the nature of electricity and magnetism, and the relationship between them. Among these were several giants in physics and other disciplinesincluding one of America's greatest founding fathers. In addition to his famous (and highly dangerous) experiment with lightning, Benjamin Franklin (1706-1790) also contributed the names "positive" and "negative" to the differing electrical charges discovered earlier by French physicist Charles Du Fay (1698-1739).

In 1785, French physicist and inventor Charles Coulomb (1736-1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the inverse of the square of the distance between objects. That attraction itself, however, resulted not from gravity, but from electrical charge, according to Coulomb.

A few years later, German mathematician Johann Karl Friedrich Gauss (1777-1855) developed a mathematical theory for finding the magnetic potential of any point on Earth, and his contemporary, Danish physicist Hans Christian Oersted (1777-1851), became the first scientist to establish the existence of a clear relationship between electricity and magnetism. This led to the foundation of electromagnetism, the branch of physics devoted to the study of electrical and magnetic phenomena.

French mathematician and physicist André Marie Ampère (1775-1836) concluded that magnetism is the result of electricity in motion, and, in 1831, British physicist and chemist Michael Faraday (1791-1867) published his theory of electromagnetic induction. This theory shows how an electrical current in one coil can set up a current in another through the development of a magnetic field. This enabled Faraday to develop the first generator, and for the first time in history, humans were able to convert mechanical energy systematically into electrical energy.

MAXWELL AND ELECTROMAGNETIC FORCE.

A number of other figures contributed along the way; but, as yet, no one had developed a "unified theory" explaining the relationship between electricity and magnetism. Then, in 1865, Scottish physicist James Clerk Maxwell (1831-1879) published a groundbreaking paper, "On Faraday's Lines of Force," in which he outlined a theory of electromagnetic forcethe total force on an electrically charged particle, which is a combination of forces due to electrical and/or magnetic fields around the particle.

Maxwell had thus discovered a type of force in addition to gravity, and this reflected a "new" type of fundamental interaction, or a basic mode by which particles interact in nature. Newton had identified the first, gravitational interaction, and in the twentieth century, two other forms of fundamental interactionstrong nuclear and weak nuclearwere identified as well.

In his work, Maxwell drew on the studies conducted by his predecessors, but added a new statement: that electrical charge is conserved. This statement, which did not contradict any of the experimental work done by the other physicists, was based on Maxwell's predictions regarding what should happen in situations of electromagnetism; subsequent studies have supported his predictions.

Electromagnetic Radiation

So far, what we have seen is the foundation for modern understanding of electricity and magnetism. This understanding grew enormously in the late nineteenth and early twentieth centuries, thanks both to the theoretical work of physicists, and the practical labors of inventors such as Thomas Alva Edison (1847-1931) and Serbian-American electrical engineer Nikola Tesla (1856-1943). But our concern in the present context is with electromagnetic radiation, of which the waves on the electromagnetic spectrum are a particularly significant example.

Energy can travel by conduction or convection, two principal means of heat transfer. But the energy Earth receives from the Sunthe energy conveyed through the electromagnetic spectrumis transferred by another method, radiation. Whereas conduction of convection can only take place where there is matter, which provides a medium for the energy transfer, radiation requires no medium. Thus, electromagnetic energy passes from the Sun to Earth through the vacuum of empty space.

ELECTROMAGNETIC WAVES.

The connection between electromagnetic radiation and electromagnetic force is far from obvious. Even today, few people not scientifically trained understand that there is a clear relationship between electricity and magnetismlet alone a connection between these and visible light. The breakthrough in establishing that connection can be attributed both to Maxwell and to German physicist Heinrich Rudolf Hertz (1857-1894).

Maxwell had suggested that electromagnetic force carried with it a certain wave phenomenon, and predicted that these waves traveled at a certain speed. In his Treatise on Electricity and Magnetism (1873), he predicted that the speed of these waves was the same as that of light186,000 mi (299,339 km) per secondand theorized that the electromagnetic interaction included not only electricity and magnetism, but light as well. A few years later, while studying the behavior of electrical currents, Hertz confirmed Maxwell's proposition regarding the wave phenomenon by showing that an electrical current generated some sort of electromagnetic radiation.

In addition, Hertz found that the flow of electrical charges could be affected by light under certain conditions. Ultraviolet light had already been identified, and Hertz shone an ultraviolet beam on the negatively charged side of a gap in a current loop. This made it easier for an electrical spark to jump the gap. Hertz could not explain this phenomenon, which came to be known as the photoelectric effect. Indeed, no one else could explain it until quantum theory was developed in the early twentieth century. In the meantime, however, Hertz's discovery of electromagnetic waves radiating from a current loop led to the invention of radio by Italian physicist and engineer Guglielmo Marconi (1874-1937) and others.

Light: Waves or Particles?

At this point, it is necessary to jump backward in history, to explain the progression of scientists' understanding of light. Advancement in this areatook place over a long period of time: at the endof the first millennium a.d., the Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039) showedthat light comes from the Sun and other self-illuminated bodiesnot, as had been believed up tothat timefrom the eye itself. Thus, studies inoptics, or the study of light and vision, werecompared to understanding of electromagnetismitselfrelatively advanced by 1666, when Newton discovered the spectrum of colors in light. AsNewton showed, colors are arranged in a sequence, and white light is a combination of allcolors.

Newton put forth the corpuscular theory of lightthat is, the idea that light is made up of particlesbut his contemporary Christiaan Huygens (1629-1695), a Dutch physicist and astronomer, maintained that light appears in the form of a wave. For the next century, adherents of Newton's corpuscular theory and of Huygens's wave theory continued to disagree. Physicists on the European continent began increasingly to accept wave theory, but corpuscular theory remained strong in Newton's homeland.

Thus, it was ironic that the physicist whose work struck the most forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Young directed a beam of light through two closely spaced pinholes onto a screen, reasoning that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interferencea wave phenomenon.

By the time of Hertz, wave theory had become dominant; but the photoelectric effect also exhibited aspects of particle behavior. Thus, for the first time in more than a century, particle theory gained support again. Yet, it was clear that light had certain wave characteristics, and this raised the questionwhich is it, a wave or a set of particles streaming through space?

The work of German physicist Max Planck (1858-1947), father of quantum theory, and of Albert Einstein (1879-1955), helped resolve this apparent contradiction. Using Planck's quantum principles, Einstein, in 1905, showed that light appears in "bundles" of energy, which travel as waves but behave as particles in certain situations. Eighteen years later, American physicist Arthur Holly Compton (1892-1962) showed that, depending on the way it is tested, light appears as either a particle or a wave. These particles he called photons.

Wave Motion and Electromagnetic Waves

The particle behavior of electromagnetic energy is beyond the scope of the present discussion, though aspects of it are discussed elsewhere. For the present purposes, it is necessary only to view the electromagnetic spectrum as a series of waves, and in the paragraphs that follow, the rudiments of wave motion will be presented in short form.

A type of harmonic motion that carries energy from one place to another without actually moving any matter, wave motion is related to oscillation, harmonicand typically periodicmotion in one or more dimensions. Oscillation involves no net movement, but only movement in place; yet individual waves themselves are oscillating, even as the overall wave pattern moves.

The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves: waves in which a uniform series of crests and troughs follow each other in regular succession. Periodic waves are divided into longitudinal and transverse waves, the latter (of which light waves are an example) being waves in which the vibration or motion is perpendicular to the direction in which the wave is moving. Unlike longitudinal waves, such as those that carry sound energy, transverse waves are fairly easy to visualize, and assume the shape that most people imagine when they think of waves: a regular up-and-down pattern, called "sinusoidal" in mathematical terms.

PARAMETERS OF WAVE MOTION.

A period (represented by the symbol T ) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength.

Frequency (abbreviated f ) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after Hertz himself: a single Hertz (the term is both singular and plural) is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.)

Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength; and, thus, it is possible to describe waves in terms of either. According to quantum theory, however, electromagnetic waves can also be described in terms of photon energy level, or the amount of energy in each photon. Thus, the electromagnetic spectrum, as we shall see, varies from relatively long-wave length, low-frequency, low-energy radio waves on the one end to extremely short-wave-length, high-frequency, high-energy gamma rays on the other.

The other significant parameter for describing a waveone mathematically independent from those so far discussedis amplitude. Defined as the maximum displacement of a vibrating material, amplitude is the "size" of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity. The amplitude of a light wave, for instance, determines the intensity of the light.

A RIGHT-HAND RULE.

Physics textbooks use a number of "right-hand rules": devices for remembering certain complex physical interactions by comparing the lines of movement or force to parts of the right hand. In the present context, a right-hand rule makes it easier to visualize the mutually perpendicular directions of electromagnetic waves, electric field, and magnetic field.

A field is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electrical field and magnetic field are simply regions in which electrical and magnetic components, respectively, of electromagnetic force are exerted.

Hold out your right hand, palm perpendicular to the floor and thumb upright. Your fingers indicate the direction that an electromagnetic wave is moving. Your thumb points in the direction of the electrical field, as does the heel of your hand: the electrical field forms a plane perpendicular to the direction of wave propagation. Similarly, both your palm and the back of your hand indicate the direction of the magnetic field, which is perpendicular both to the electrical field and the direction of wave propagation.

The Electromagnetic Spectrum

As stated earlier, an electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with mutually perpendicular electrical and magnetic fields that emanate from it.

The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. Though each occupies a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply "blurs" into another.

FREQUENCY RANGE OF THE ELECTROMAGNETIC SPECTRUM.

The range of frequencies for waves in the electromagnetic spectrum is from approximately 102 Hz to more than 1025 Hz. These numbers are an example of scientific notation, which makes it possible to write large numbers without having to include a string of zeroes. Without scientific notation, the large numbers used for discussing properties of the electromagnetic spectrum can become bewildering.

The first number given, for extremely low-frequency radio waves, is simple enough100but the second would be written as 1 followed by 25 zeroes. (A good rule of thumb for scientific notation is this: for any power n of 10, simply attach that number of zeroes to 1. Thus 106 is 1 followed by 6 zeroes, and so on.) In any case, 1025 is a much simpler figure than 10,000,000,000,000,000,000,000,000or 10 trillion trillion. As noted earlier, gigahertz, or units of 1 billion Hertz, are often used in describing extremely high frequencies, in which case the number is written as 1016 GHz. For simplicity's sake, however, in the present context, the simple unit of Hertz (rather than kilo-, mega-, or gigahertz) is used wherever it is convenient to do so.

WAVELENGTHS ON THE ELECTROMAGNETIC SPECTRUM.

The range of wavelengths found in the electromagnetic spectrum is from about 108 centimeters to less than 1015 centimeters. The first number, equal to 1 million meters (about 621 mi), obviously expresses a great length. This figure is for radio waves of extremely low frequency; ordinary radio waves of the kind used for actual radio broadcasts are closer to 105 centimeters (about 328 ft).

For such large wavelengths, the use of centimeters might seem a bit cumbersome; but, as with the use of Hertz for frequencies, centimeters provide a simple unit that can be used to measure all wavelengths. Some charts of the electromagnetic spectrum nonetheless give figures in meters, but for parts of the spectrum beyond microwaves, this, too, can become challenging. The ultra-short wavelengths of gamma rays, after all, are equal to one-trillionth of a centimeter. By comparison, the angstroma unit so small it is used to measure the diameter of an atomis 10 million times as large.

ENERGY LEVELS ON THE ELECTROMAGNETIC SPECTRUM.

Finally, in terms of photon energy, the unit of measurement is the electron volt (eV), which is used for quantifying the energy in atomic particles. The range of photon energy in the electromagnetic spectrum is from about 1013 to more than 1010 electron volts. Expressed in terms of joules, an electron volt is equal to 1.6 · 1019 J.

To equate these figures to ordinary language would require a lengthy digression; suffice it to say that even the highest ranges of the electromagnetic spectrum possess a small amount of energy in terms of joules. Remember, however, that the energy level identified is for a photona light particle. Again, without going into a great deal of detail, one can just imagine how many of these particles, which are much smaller than atoms, would fit into even the smallest of spaces. Given the fact that electromagnetic waves are traveling at a speed equal to that of light, the amount of photon energy transmitted in a single second is impressive, even for the lower ranges of the spectrum. Where gamma rays are concerned, the energy levels are positively staggering.

REAL-LIFE APPLICATIONS

The Radio Sub-Spectrum

Among the most familiar parts of the electromagnetic spectrum, in modern life at least, is radio. In most schematic representations of the spectrum, radio waves are shown either at the left end or the bottom, as an indication of the fact that these are the electromagnetic waves with the lowest frequencies, the longest wavelengths, and the smallest levels of photon energy. Included in this broad sub-spectrum, with frequencies up to about 107 Hertz, are long-wave radio, short-wave radio, and microwaves. The areas of communication affected are many: broadcast radio, television, mobile phones, radarand even highly specific forms of technology such as baby monitors.

Though the work of Maxwell and Hertz was foundational to the harnessing of radio waves for human use, the practical use of radio had its beginnings with Marconi. During the 1890s, he made the first radio transmissions, and, by the end of the century, he had succeeded in transmitting telegraph messages across the Atlantic Oceana feat which earned him the Nobel Prize for physics in 1909.

Marconi's spark transmitters could send only coded messages, and due to the broad, long-wave length signals used, only a few stations could broadcast at the same time. The development of the electron tube in the early years of the twentieth century, however, made it possible to transmit narrower signals on stable frequencies. This, in turn, enabled the development of technology for sending speech and music over the airwaves.

Broadcast Radio

THE DEVELOPMENT OF AM AND FM.

A radio signal is simply a carrier: the process of adding informationthat is, complex sounds such as those of speech or musicis called modulation. The first type of modulation developed was AM, or amplitude modulation, which Canadian-American physicist Reginald Aubrey Fessenden (1866-1932) demonstrated with the first United States radio broadcast in 1906. Amplitude modulation varies the instantaneous amplitude of the radio wave, a function of the radio station's power, as a means of transmitting information.

By the end of World War I, radio had emerged as a popular mode of communication: for the first time in history, entire nations could hear the same sounds at the same time. During the 1930s, radio became increasingly important, both for entertainment and information. Families in the era of the Great Depression would gather around large "cathedral radios"so named for their size and shapeto hear comedy programs, soap operas, news programs, and speeches by important public figures such as President Franklin D. Roosevelt.

Throughout this eraindeed, for more than a half-century from the end of the first World War to the height of the Vietnam Conflict in the mid-1960sAM held a dominant position in radio. This remained the case despite a number of limitations inherent in amplitude modulation: AM broadcasts flickered with popping noises from lightning, for instance, and cars with AM radios tended to lose their signal when going under a bridge. Yet, another mode of radio transmission was developed in the 1930s, thanks to American inventor and electrical engineer Edwin H. Armstrong (1890-1954). This was FM, or frequency modulation, which varied the radio signal's frequency rather than its amplitude.

Not only did FM offer a different type of modulation; it was on an entirely different frequency range. Whereas AM is an example of a long-wave radio transmission, FM is on the microwave sector of the electromagnetic spectrum, along with television and radar. Due to its high frequency and form of modulation, FM offered a "clean" sound as compared with AM. The addition of FM stereo broadcasts in the 1950s offered still further improvements; yet despite the advantages of FM, audiences were slow to change, and FM did not become popular until the mid-to late 1960s.

SIGNAL PROPAGATION.

AM signals have much longer wavelengths, and smaller frequencies, than do FM signals, and this, in turn, affects the means by which AM signals are propagated. There are, of course, much longer radio wavelengths; hence, AM signals are described as intermediate in wavelength. These intermediate-wavelength signals reflect off highly charged layers in the ionosphere between 25 and 200 mi (40-332 km) above Earth's surface. Short-wave-length signals, such as those of FM, on the other hand, follow a straight-line path. As a result, AM broadcasts extend much farther than FM, particularly at night.

At a low level in the ionosphere is the D layer, created by the Sun when it is high in the sky. The D layer absorbs medium-wavelength signals during the day, and for this reason, AM signals do not travel far during daytime hours. After the Sun goes down, however, the D layer soon fades, and this makes it possible for AM signals to reflect off a much higher layer of the ionosphere known as the F layer. (This is also sometimes known as the Heaviside layer, or the Kennelly-Heaviside layer, after English physicist Oliver Heaviside and British-American electrical engineer Arthur Edwin Kennelly, who independently discovered the ionosphere in 1902.) AM signals "bounce" off the F layer as though it were a mirror, making it possible for a listener at night to pick up a signal from halfway across the country.

The Sun has other effects on long-wave and intermediate-wave radio transmissions. Sunspots, or dark areas that appear on the Sun in cycles of about 11 years, can result in a heavier buildup of the ionosphere than normal, thus impeding radio-signal propagation. In addition, occasional bombardment of Earth by charged particles from the Sun can also disrupt transmissions.

Due to the high frequencies of FM signals, these do not reflect off the ionosphere; instead, they are received as direct waves. For this reason, an FM station has a fairly short broadcast range, and this varies little with regard to day or night. The limited range of FM stations as compared to AM means that there is much less interference on the FM dial than for AM.

Distribution of Radio Frequencies

In the United States and most other countries, one cannot simply broadcast at will; the airwaves are regulated, and, in America, the governing authority is the Federal Communications Commission (FCC). The FCC, established in 1934, was an outgrowth of the Federal Radio Commission, founded by Congress seven years earlier. The FCC actually "sells air," charging companies a fee to gain rights to a certain frequency. Those companies may in turn sell that air to others for a profit.

At the time of the FCC's establishment, AM was widely used, and the federal government assigned AM stations the frequency range of 535 kHz to 1.7 MHz. Thus, if an AM station today is called, for instance, "AM 640," this means that it operates at 640 kHz on the dial. The FCC assigned the range of 5.9 to 26.1 MHz to short-wave radio, and later the area of 26.96 to 27.41 MHz to citizens' band (CB) radio. Above these are microwave regions assigned to television stations, as well as FM, which occupies the range from 88 to 108 MHz.

The organization of the electromagnetic spectrum's radio frequencieswhich, of course, is an entirely arbitrary, humanmade processis fascinating. It includes assigned frequencies for everything from garage-door openers to deep-space radio communications. The FCC recognizes seven divisions of radio carriers, using a system that is not so much based on rational rules as it is on the way that the communications industries happened to develop over time.

THE SEVEN FCC DIVISIONS.

Most of what has so far been described falls under the heading of "Public Fixed Radio Services": AM and FM radio, other types of radio such as shortwave, television, various other forms of microwave broadcasting, satellite systems, and communication systems for federal departments and agencies. "Public Mobile Services" include pagers, air-to-ground service (for example, aircraft-to-tower communications), offshore service for sailing vessels, and rural radio-telephone service. "Commercial Mobile Radio Services" is the realm of cellular phones, and "Personal Communications Service" that of the newer wireless technology that began to challenge cellular for market dominance in the late 1990s.

"Private Land Mobile Radio Service" (PMR) and "Private Operational-Fixed Microwave Services" (OFS) are rather difficult to distinguish, the principal difference being that the former is used exclusively by profit-making businesses, and the latter mostly by nonprofit institutions. An example of PMR technology is the dispatching radios used by taxis, but this is only one of the more well-known forms of internal electronic communications for industry. For instance, when a film production company is shooting a picture and the director needs to speak to someone at the producer's trailer a mile away, she may use PMR radio technology. OFS was initially designated purely for nonprofit use, and is used often by schools; but banks and other profit-making institutions often use OFS because of its low cost.

Finally, there is the realm of "Personal Radio Services," created by the FCC in 1992. This branch, still in its infancy, will probably one day include video-on-demand, interactive polling, online shopping and banking, and other activities classified under the heading of Interactive Video and Data Services, or IVDS. Unlike other types of video technology, these will all be wireless, and, therefore, represent a telecommunications revolution all their own.

Microwaves

MICROWAVE COMMUNICATION.

Though microwaves are treated separately from radio waves, in fact, they are just radio signals of a very short wavelength. As noted earlier, FM signals are actually carried on microwaves, and, as with FM in particular, microwave signals in general are very clear and very strong, but do not extend over a great geographical area. Nor does microwave include only high-frequency radio and television; in fact, any type of information that can be transmitted via telephone wires or coaxial cables can also be sent via a microwave circuit.

Microwaves have a very narrow, focused beam: thus, the signal is amplified considerably when an antenna receives it. This phenomenon, known as "high antenna gain," means that microwave transmitters need not be highly powerful to produce a strong signal. To further the reach of microwave broadcasts, transmitters are often placed atop mountain peaks, hilltops, or tall buildings. In the past, a microwave-transmitting network such as NBC (National Broadcasting Company) or CBS (Columbia Broadcasting System) required a network of ground-based relay stations to move its signal across the continent. The advent of satellite broadcasting in the 1960s, however, changed much about the way signals are beamed: today, networks typically replace, or at least augment, ground-based relays with satellite relays.

The first worldwide satellite TV broadcast, in the summer of 1967, featured the Beatles singing their latest song "All You Need Is Love." Due to the international character of the broadcast, with an estimated 200 million viewers, John Lennon and Paul McCartney wrote a song with simple, universal lyrics, and the result was just another example of electronic communication uniting large populations. Indeed, the phenomenon of rock music, and of superstardom as people know it today, would be impossible without many of the forms of technology discussed here. Long before the TV broadcast, the Beatles had come to fame through the playing of their music on the radio wavesand, thus, they owed much to Maxwell, Hertz, and Marconi.

MICROWAVE OVENS.

The same microwaves that transmit FM and television signalsto name only the most obviously applications of microwave for communicationcan also be harnessed to cook food. The microwave oven, introduced commercially in 1955, was an outgrowth of military technology developed a decade before.

During World War II, the Raytheon Manufacturing Company had experimented with a magnetron, a device for generating extremely short-wavelength radio signals as a means of improving the efficiency of military radar. While working with a magnetron, a technician named Percy Spencer was surprised to discover that a candy bar in his pocket had melted, even though he had not felt any heat. This led him to considering the possibilities of applying the magnetron to peacetime uses, and a decade later, Raytheon's "radar range" hit the market.

Those early microwave ovens had none of varied power settings to which modern users of the microwavefound today in two-thirds of all American homesare accustomed. In the first microwaves, the only settings were "on" and "off," because there were only two possible adjustments: either the magnetron would produce, or not produce, microwaves. Today, it is possible to use a microwave for almost anything that involves the heating of food that contains waterfrom defrosting a steak to popping popcorn.

As noted much earlier, in the general discussion of electromagnetic radiation, there are three basic types of heat transfer: conduction, convection, and radiation. Without going into too much detail here, conduction generally involves heat transfer between molecules in a solid; convection takes place in a fluid (a gas such as air or a liquid such as water); and radiation, of course, requires no medium.

A conventional oven cooks through convection, though conduction also carries heat from the outer layers of a solid (for example, a turkey) to the interior. A microwave, on the other hand, uses radiation to heat the outer layers of the food; then conduction, as with a conventional oven, does the rest. The difference is that the microwave heats only the foodor, more specifically, the water, which then transfers heat throughout the item being heatedand not the dish or plate. Thus, many materials, as long as they do not contain water, can be placed in a microwave oven without being melted or burned. Metal, though it contains no water, is unsafe because the microwaves bounce off the metal surfaces, creating a microwave buildup that can produce sparks and damage the oven.

In a microwave oven, microwaves emitted by a small antenna are directed into the cooking compartment, and as they enter, they pass a set of turning metal fan blades. This is the stirrer, which disperses the microwaves uniformly over the surface of the food to be heated. As a microwave strikes a water molecule, resonance causes the molecule to align with the direction of the wave. An oscillating magnetron causes the microwaves to oscillate as well, and this, in turn, compels the water molecules to do the same. Thus, the water molecules are shifting in position several million times a second, and this vibration generates energy that heats the water.

Radio Waves for Measurement and Ranging

RADAR.

Radio waves can be used to send communication signals, or even to cook food; they can also be used to find and measure things. One of the most obvious applications in this regard is radar, an acronym for RA dio D etection A nd R anging.

Radio makes it possible for pilots to "see" through clouds, rain, fog, and all manner of natural phenomenanot least of which is darkness. It can also identify objects, both natural and manmade, thus enabling a peacetime pilot to avoid hitting another craft or the side of a mountain. On the other hand, radar may help a pilot in wartime to detect the presence of an enemy. Nor is radar used only in the skies, or for military purposes, such as guiding missiles: on the ground, it is used to detect the speeds of objects such as automobiles on an interstate highway, as well as to track storms.

In the simplest model of radar operation, the unit sends out microwaves toward the target, and the waves bounce back off the target to the unit. Though the speed of light is reduced somewhat, due to the fact that waves are traveling through air rather than through a vacuum, it is, nonetheless, possible to account for this difference. Hence, the distance to the target can be calculated using the simple formula d = vt, where d is distance, v is velocity, and t is time.

Typically, a radar system includes the following: a frequency generator and a unit for controlling the timing of signals; a transmitter and, as with broadcast radio, a modulator; a duplexer, which switches back and forth between transmission and reception mode; an antenna; a receiver, which detects and amplifies the signals bounced back to the antenna; signal and data processing units; and data display units. In a monostatic unitone in which the transmitter and receiver are in the same locationthe unit has to be continually switched between sending and receiving modes. Clearly, a bistatic unitone in which the transmitter and receiver antennas are at different locationsis generally preferable; but on an airplane, for instance, there is no choice but to use a monostatic unit.

In order to determine the range to a targetwhether that target be a mountain, an enemy aircraft, or a stormthe target itself must first be detected. This can be challenging, because only a small portion of the transmitted pulse comes back to the receiving antenna. At the same time, the antenna receives reflections from a number of other objects, and it can be difficult to determine which signal comes from the target. For an aircraft in a wartime situation, these problems are compounded by the use of enemy countermeasures such as radar "jamming." Still another difficulty facing a military flyer is the fact that the use of radar itselfthat is, the transmission of microwavesmakes the aircraft detectable to opposing forces.

TELEMETRY.

Telemetry is the process of making measurements from a remote location and transmitting those measurements to receiving equipment. The earliest telemetry systems, developed in the United States during the 1880s, monitored the distribution and use of electricity in a given region, and relayed this information back to power companies using telephone lines. By the end of World War I, electric companies used the power lines themselves as information relays, and though such electrical telemetry systems remain in use in some sectors, most modern telemetry systems apply radio signals.

An example of a modern telemetry application is the use of an input device called a transducer to measure information concerning an astronaut's vital signs (heartbeat, blood pressure, body temperature, and so on) during a manned space flight. The transducer takes this information and converts it into an electrical impulse, which is then beamed to the space monitoring station on Earth. Because this signal carries information, it must be modulated, but there is little danger of interference with broadcast transmissions on Earth. Typically, signals from spacecraft are sent in a range above 1010 Hz, far above the frequencies of most microwave transmissions for commercial purposes.

Light: Invisible, Visible, and Invisible Again

Between about 1013 and 1017 Hz on the electromagnetic spectrum is the range of light: infrared, visible, and ultraviolet. Light actually constitutes a small portion of the spectrum, and the area of visible light is very small indeed, extending from about 4.3 · 1014 to 7.5 · 1014 Hz. The latter, incidentally, is another example of scientific notation: not only is it easier not to use a string of zeroes, but where a coefficient or factor (for example, 4.3 or 7.5) is other than a multiple of 10, it is preferable to use what are called significant figuresusually a single digit followed by a decimal point and up to 3 decimal places.

Infrared light lies just below visible light in frequency, and this is easy to remember because of the name: red is the lowest in frequency of all the colors. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Visible light itself, by far the most familiar part of the spectrumespecially prior to the age of radio communicationsis discussed in detail elsewhere.

INFRARED LIGHT.

Though we cannot see infrared light, we feel it as heat. German-English astronomer William Herschel (1738-1822), first scientist to detect infrared radiation from the Sun, demonstrated its existence in 1800 by using a thermometer. Holding a prism, a three-dimensional glass shape used for diffusing beams of light, he directed a beam of sunlight toward the thermometer, which registered the heat of the infrared rays.

Eighty years later, English scientist Sir William Abney (1843-1920) developed infrared photography, a method of capturing infrared radiation, rather than visible light, on film. By the mid-twentieth century, infrared photography had come into use for a variety of purposes. Military forces, for instance, may use infrared to detect the presence of enemy troops. Medicine makes use of infrared photography for detecting tumors, and astronomers use infrared to detect stars too dim to be seen using ordinary visible light.

The uses of infrared imaging in astronomy, as a matter of fact, are many. The development in the 1980s of infrared arrays, two-dimensional grids which produce reliable images of infrared phenomena, revolutionized infrared astronomy. Because infrared penetrates dust much more easily than does visible light, infrared astronomy makes it easier to see regions of the universe where starsformed from collapsing clouds of gas and dustare in the process of developing. Because hydrogen molecules emit infrared radiation, infrared astronomy helps provide clues regarding the distribution of this highly significant chemical element throughout the universe.

ULTRAVIOLET LIGHT.

Very little of the Sun's ultraviolet light penetrates Earth's atmospherea fortunate thing, since ultraviolet (UV) radiation can be very harmful to human skin. A suntan, as a matter of fact, is actually the skin's defense against these harmful UV rays. Due to the fact that Earth is largely opaque, or resistant, to ultraviolet light, the most significant technological applications of UV radiation are found in outer space.

In 1978 the United States, in cooperation with several European space agencies, launched the International Ultraviolet Explorer (IUE), which measured the UV radiation from tens of thousands of stars, nebulae, and galaxies. Despite the progress made with IUE, awareness of its limitationsincluding a mirror of only 17 in (45 cm) on the telescope itselfled to the development of a replacement in 1992.

This was the Extreme Ultraviolet Explorer (EUVE), which could observe UV phenomena over a much higher range of wavelengths than those observed by IUE. In addition, the Hubble Space Telescope, launched by the United States in 1990, includes a UV instrument called the Goddard High Resolution Spectrograph. With a mirror measuring 8.5 ft (2.6 m), it is capable of observing objects much more faint than those detected earlier by IUE.

Ultraviolet astronomy is used to study the winds created by hot stars, as well as stars still in the process of forming, and even stars that are dying. It is also useful for analyzing the densely packed, highly active sectors near the centers of galaxies, where both energy and temperatures are extremely high.

X Rays

Though they are much higher in frequency than visible lightwith wavelengths about 1,000 times shorter than for ordinary light raysx rays are a familiar part of modern life due to their uses in medicine. German scientist Wilhelm Röntgen (1845-1923) developed the first x-ray device in 1895, and, thus, the science of using xray machines is called roentgenology.

The new invention became a curiosity, with carnivals offering patrons an opportunity to look at the insides of their hands. And just as many people today fear the opportunities for invasion of privacy offered by computer technology, many at the time worried that x rays would allow robbers and peeping toms to look into people's houses. Soon, however, it became clear that the most important application of x rays lay in medicine.

HOW A MEDICAL X-RAY MACHINE WORKS.

Due to their very short wavelengths, x rays can pass through substances of low densityfor example, fat and other forms of soft tissuewithout their movement being interrupted. But in materials of higher density, such as bone, atoms are packed closely together, and this provides x rays with less space through which to travel. As a result, x-ray images show dark areas where the rays traveled completely through the target, and light images of dense materials that blocked the movement of the rays.

Medical x-ray machines are typically referred to either as "hard" or "soft." Soft x rays are the ones with which most people are more familiar. Operating at a relatively low frequency, these are used to photograph bones and internal organs, and provided the patient does not receive prolonged exposure to the rays, they cause little damage. Hard x rays, on the other hand, are designed precisely to cause damagenot to the patient, but to cancer cells. Because they use high voltage and high-frequency rays, hard x rays can be quite dangerous to the patient as well.

OTHER APPLICATIONS.

X-ray crystallography, developed in the early twentieth century, is devoted to the study of the interference patterns produced by x rays passing through materials that are crystalline in Structure. Each of these discoveries, in turn, transformed daily life: insulin, by offering hope to diabetics, penicillin, by providing a treatment for a number of previously fatal illnesses, and DNA, by enabling scientists to make complex assessments of genetic information.

In addition to the medical applications, the scanning capabilities of x-ray machines make them useful for security. A healthy person receives an x ray at a doctor's office only once in a while; but everyone who carries items past a certain point in a major airport must submit to x-ray security scanning. If one is carrying a purse or briefcase, for instance, this is placed on a moving belt and subjected to scanning by a low-power device that can reveal the contents.

Gamma Rays

At the furthest known reaches of the electromagnetic spectrum are gamma rays, ultra high-frequency, high-energy, and short-wavelength forms of radiation. Human understanding of gamma rays, including the awesome powers they contain, is still in its infancy.

In 1979, a wave of enormous energy passed over the Solar System. Though its effects on Earth were negligible, instruments aboard several satellites provided data concerning an enormous quantity of radiation caused by gamma rays. As to the source of the rays themselves, believed to be a product of nuclear fusion on some other body in the universe, scientists knew nothing.

The Compton Gamma Ray Observatory Satellite, launched by NASA (National Aeronautics and Space Administration) in 1991, detected a number of gamma-ray bursts over the next two years. The energy in these bursts was staggering: just one of these, scientists calculated, contained more than a thousand times as much energy as the Sun will generate in its entire lifetime of 10 billion years.

Some astronomers speculate that the source of these gamma-ray bursts may ultimately be a distant supernova, or exploding star. If this is the case, scientists may have found the supernova; but do not expect to see it in the night sky. It is not known just how long ago it exploded, but its light appeared on Earth some 340,000 years ago, and during that time it was visible in daylight for more than two years. So great was its power that the effects of this stellar phenomenon are still being experienced.

WHERE TO LEARN MORE

Branley, Franklyn Mansfield. The Electromagnetic Spectrum: Key to the Universe. Illustrated by Leonard D. Dank. New York: Crowell, 1979.

Electromagnetic Spectrum (Web site). <http://www.jsc.mil/images/speccht.jpg> (April 25, 2001).

The Electromagnetic Spectrum (Web site). <http://library.thinkquest.org/11119/index.htm> (April 30, 2001).

The Electromagnetic Spectrum (Web site). <http://www.ospi.wednet.edu:8001/curric/space/lfs/emspectr.html> (April 30, 2001).

Electromagnetic Spectrum/NASA: National Aeronautics and Space Administration (Web site). <http://imagine.gsfc.nasa.gov/docs/science/know_11/emspectrum.html> (April 30, 2001).

"How the Radio Spectrum Works." How Stuff Works (Web site). <http://www.howstuffworks.com/radio~spectrum.htm> (April 25, 2001).

Internet Resources for Sound and Light (Web site). <http://electro.sau.edu/SLResources.html> (April 25, 2001).

Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997.

"Radio Electronics Pages" ePanorama.net (Web site). <http://www.epanorama.net/radio.html> (April 25, 2001).

Skurzynski, Gloria. Waves: The Electromagnetic Universe. Washington, D.C.: National Geographic Society, 1996.

KEY TERMS

AMPLITUDE:

The maximum displacement of a vibrating material. In wave motion, amplitude is the "size" of a wave, an indicator of the energy and intensity of the wave.

CYCLE:

One complete oscillation. In wave motion, this is equivalent to the movement of a wave from trough to crest and back to trough.

ELECTROMAGNETIC FORCE:

The total force on an electrically charged particle, which is a combination of forces due toelectrical and/or magnetic fields around the particle. Electromagnetic force reflects electromagnetic interaction, one of the four fundamental interactions in nature.

ELECTROMAGNETIC SPECTRUM:

The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energylevels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays.

ELECTROMAGNETIC WAVE:

A transverse wave with electrical and magnetic fields that emanate from it. The directions of these fields are perpendicular to one another, and both are perpendicular to the line of propagation for the wave itself.

ELECTROMAGNETISM:

The branch of physics devoted to the study of electrical and magnetic phenomena.

FIELD:

A region of space in which it is possible to define the physical properties of each point in the region at any given moment in time.

FREQUENCY:

In wave motion, frequency is the number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength. Measured in Hertz, frequency is mathematically related to wave speed, wavelength, and period.

FUNDAMENTAL INTERACTION:

The basic mode by which particles interact. There are four known fundamental interactions in nature: gravitational, electromagnetic, strong nuclear, and weaknuclear.

HARMONIC MOTION:

The repeated movement of a particle about a position of equilibrium, or balance.

HERTZ:

A unit for measuring frequency, named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894). High frequencies are expressed in terms of kilohertz (kHz; 103 or1,000 cycles per second); megahertz (MHz;106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.)

INTENSITY:

Intensity is the rate at which a wave moves energy per unit of cross-sectional area.

OSCILLATION:

A type of harmonic motion, typically periodic, in one or more dimensions.

PERIOD:

For wave motion, a period is the amount of time required to complete one full cycle. Period is mathematically related to frequency, wavelength, and wave speed.

PERIODIC MOTION:

Motion that is repeated at regular intervals. These intervals are known as periods.

PERIODIC WAVE:

A wave in which a uniform series of crests and troughs follow one after the other in regular succession.

PHOTON:

A particle of electromagnetic radiation carrying a specific amount of energy, measured in electron volts (eV). For parts of the electromagnetic spectrum with a low frequency and long wavelength, photon energy is relatively low; but for parts with a high frequency and shortwave length, the value of photon energy is very high.

PROPAGATION:

The act or state of traveling from one place to another.

RADIATION:

The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy, via the electromagnetic spectrum, by means of radiation.

SCIENTIFIC NOTATION:

A methodused by scientists for writing extremely large numbers. This usually involves a coefficient, or factor, of a single digit followed by a decimal point and up to three decimalplaces, multiplied by 10 to a given exponent. Thus, instead of writing 75,120,000, the preferred scientific notation is 7.512 · 107. To visualize the value of very largemultiples of 10, it is helpful to remember that the value of 10 raised to any power n is the same as 1 followed by that number of zeroes. Hence 1025, for instance, is simply 1 followed by 25 zeroes.

TRANSVERSE WAVE:

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving.

WAVELENGTH:

The distance between a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. Wavelength, symbolized λ (the Greek letter lambda) is mathematically related to wave speed, period, and frequency.

WAVE MOTION:

A type of harmonic motion that carries energy from one place to another without actually moving anymatter.

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Electromagnetic Spectrum

Electromagnetic Spectrum

LARRY GILMAN

The electromagnetic spectrum consists of all the frequencies at which electromagnetic waves can occur, ordered from zero to infinity. Radio waves, visible light, and x rays are examples of electromagnetic waves at different frequencies. Every part of the electromagnetic spectrum is exploited for some form of military, security, or espionage activity; the entire spectrum is also key to science and industry.

Basic Physics

Electromagnetic waves have been known since the midnineteenth century, when their behavior was first described by the equations of Scottish physicist James Clerk Maxwell (18311879). Electromagnetic waves, according to Maxwell's equations, are generated whenever an electrical charge (e.g., an electron) is accelerated, that is, changes its direction of motion, its speed, or both. An electromagnetic wave is so named because it consists of an electric and a magnetic field propagating together through space. As the electric field varies with time, it renews the magnetic field; as the magnetic field varies, it renews the electric field. The two components of the wave, which always point at right angles both to each other and to their direction of motion, are thus mutually sustaining, and form a wave which moves forward through empty space indefinitely.

The rate at which energy is periodically exchanged between the electric and magnetic components of a given electromagnetic wave is the frequency, [.nu], of that wave and has units of cycles per second or Hertz (Hz); the linear distance between the wave's peaks is termed its wavelength, [.lambda], and has units of length (e.g., meters). The speed at which a wave travels is the product of its wavelength and its frequency, V = [.nu][.lambda]; in the case of electromagnetic waves, Maxwell's equations require that this velocity equal the speed of light, c (>186,000 miles per second [300,000 km/sec]). Since the velocity of all electromagnetic waves is fixed, the wavelength [.lambda] of an electromagnetic wave always determines its frequency [.nu], or vice versa, by the relationship c = [.nu][.lambda] The higher the frequency (i.e., the shorter the wavelength) of an electromagnetic wave, the higher in the spectrum it is said to be. Since a wave cannot have a frequency less than zero, the spectrum is bound by zero at its lower end. In theory, it has no upper limit.

Electromagnetic waves and matter. All atoms and molecules at temperatures above absolute zero radiate electromagnetic waves at specific frequencies that are determined by the details of their internal structure. In quantum physics, this radiation must often be described as consisting of particles called photons rather than as waves; however, this article will restrict itself to the classical (continuous-wave) treatment of electromagnetic radiation, which is adequate for most technological purposes.

Not only do atoms and molecules radiate electromagnetic waves at certain frequencies, they can absorb them at the same frequencies. All material objects, therefore, are continuously absorbing and radiating electromagnetic waves having various frequencies, thus exchanging energy with other objects, near and far. This makes it possible to observe objects at a distance by detecting the electromagnetic waves that they radiate or reflect, or to affect them in various ways by beaming electromagnetic waves at them. These facts make the manipulation of electromagnetic waves at various frequencies (i.e., from various parts of the electromagnetic spectrum) fundamental to many fields of technology and science, including radio communication, radar, infrared sensing, visible-light imaging, lasers, x rays, astronomy, and more.

The Spectrum

The spectrum has been divided by physicists into a number of frequency ranges or bands denoted by convenient names. The points at which these bands begin and end do not correspond to shifts in the physics of electromagnetic radiation; rather, they reflect the importance of different frequency ranges for human purposes. Below, the various parts of the spectrum are named in order, lowestfrequency to highest-frequency, and their properties described.

Radio. Radio waves are typically produced by time-varying electrical currents in relatively large objects (i.e., at least centimeters across). This category of electromagnetic waves extends from the lowest-frequency, longest-wave-length electromagnetic waves up into the gigahertz (GHz; billions of cycles per second) range. The U.S. government officially allocates sub-bands of the radio frequency spectrum to various military and commercial purposes from 9 × 103 Hz to 3 × 1011 Hz, dividing this part of the spectrum up into over 450 non-overlapping frequency bands. These bands are exploited by different users and technologies: for example, broadcast FM is transmitted using frequencies on the order of 106 Hz, while television signals are transmitted using frequencies on the order of 108 Hz (about a hundred times higher). In general, higher-frequency signals can be used to transmit lower-frequency information, but not the reverse; thus a voice signal with a maximum frequency content of 20 kHz (kilohertz, thousands of Hertz) can, if desired, be transmitted on a signal centered in the Ghz range, but it is impossible to transmit a television signal over a broadcast FM station. From 109 to 3 × 1011 Hz, radio waves are termed microwaves; these are used for high-speed communications links, heating food, radar, and electromagnetic weapons, that is, devices designed to irritate or injure people or to disable enemy devices. The microwave frequencies used for communications and radar are subdivided still further into frequency bands with special designations, such as "X BAND" and "Y band." Microwave radiation from the Big Bang, the cosmic explosion in which the Universe originated, pervades all of space.

Infrared. Electromagnetic waves from approximately 1012 to 5 µlt 1014 Hz are termed infrared radiation. The word infrared means "below red," and is assigned to these waves because their frequencies are just below those of red light, the lowest-frequency light visible to human beings. Infrared radiation is typically produced by molecular vibrations and rotations (i.e., heat) and causes or accelerates such motions in the molecules of objects that absorb it; it is, therefore, perceived by the body through the increased warmth of skin exposed to it. Since all objects above absolute zero emit infrared radiation, electronic devices sensitive to infrared can form images even in the absence of visible light. Because of their ability to "see" at night, imaging devices that electronically create visible images from infrared light are important in security systems, on the battlefield, and in observations of the Earth from space for both scientific and military purposes.

Visible. Visible light consists of elecromagnetic waves with frequencies in the 4.3 × 1014 to 7.5 × 1014 Hz range. Waves in this narrow band are typically produced by rearrangements (orbital shifts) in the outer electrons of atoms. Most of the energy in the sunlight that reaches the Earth's surface consists of electromagnetic waves in this narrow frequency range; our eyes have therefore evolved to be sensitive to this band of the electromagnetic spectrum. Photovoltaic cellselectronic devices which turn incident electromagnetic radiation into electricityare also designed to work primarily in this band, and for the same reason. Because half the Earth is liberally illuminated by visible light at all times, this band of the spectrum, though narrow (less than an octave), is essential to thousands of applications, including all forms of natural and many forms of mechanical vision.

Ultraviolet. Ultraviolet light consists of electromagnetic waves with frequencies in the 7.5 × 1014 to 1016 Hz range. It is typically produced by rearrangements in the outer and intermediate electrons of atoms. Ultraviolet light is invisible, but can cause chemical changes in many substances: for living things, consequences of these chemical changes can include skin burns, blindness, or cancer. Ultraviolet light can also cause some substances to give off visible light (flouresce), a property useful for mineral detection, art-forgery detection, and other applications. Various industrial processes employ ultraviolet light, including photolithography, in which patterned chemical changes are produced rapidly over an entire film or surface by projecting patterned ultraviolet light onto it. Most ultraviolet light from the sun is absorbed by a thin layer of ozone (O3 in the stratosphere, making the Earth's surface much more hospitable to life than it would be otherwise; some chemicals produced by human industry (e.g., chlorfluorocarbons) destroy ozone, threatening this protective layer.

X rays. Electromagnetic waves with frequences from about 1016 to 1019 Hz are termed x rays. x rays are typically produced by rearrangements of electrons in the innermost orbitals of atoms. When absorbed, they are capable of ejecting electrons entirely from atoms and thus ionizing them (i.e., causing them to have a net positive electric charge). Ionization is destructive to living tissues because ions may abandon their original molecular bonds and form new ones, altering the structure of a DNA molecule or some other aspect of cell chemistry. However, x rays are useful in medical diagnosis and in security systems (e.g., airline luggage scanners) because they can pass entirely through many solid objects; both traditional contrast images of internal structure (often termed "x RAYS" for short) and modern computerized axial tomography images, which give much more information, depend on the penetrating power of x rays. x rays are produced in large quantities by nuclear explosions (as are electromagnetic waves at all other frequencies above the radio band), and have been proposed for use in a space-based ballistic-missile defense system as follows: X-rays emitted by an orbital nuclear explosion would stimulate coherent, highly-directional x-ray emission (x-ray lasing) in special fibers placed next to the warhead that had been pre-aimed at ballistic warheads arcing through space. The resulting xray laser bursts would disable the warheads or knock them off course. There are, however, many technical and political problems with such a scheme, and its feasibility has never been demonstrated.

Gamma rays. All electromagnetic waves above about 3 × 1019 Hz are termed gamma rays ([.gamma] rays). Gamma rays are typically produced by rearrangements of particles in atomic nuclei. A nuclear explosion produces large quantities of gamma radiation, which is both directly and indirectly destructive of life. By interacting with the Earth's magnetic field, gamma rays from a high-altitude nuclear explosion can cause an intense pulse of radio waves termed an electromagnetic pulse (EMP). EMP may be powerful enough to burn out unprotected electronics on the ground over a wide area; most military hardware is therefore "hardened" against EMP to some degree, although hardening standards vary from one sector of the military to another.

Radio-frequency spectrum allocation. Radio waves present a unique regulatory problem, for only one broadcaster at a particular frequency can function in a given area. (Signals from overlapping same-frequency broadcasts would be received simultaneously by antennas, interfering with each other.) Throughout the world, therefore, governments regulate the radio portion of the electromagnetic spectrum, a process termed spectrum allocation. In the U.S., since the passage of the Communications Act of 1934, the radio spectrum has been deemed a public resource. Individual private broadcasters are given licenses allowing them to use specific portions of this resource, that is, specific sub-bands of the radio spectrum. The United States Commerce Department's National Telecommunications and Information Administration (NTIA) and FCC (Federal Communications Commission) oversee the spectrum allocation process, which is subject to intense lobbying by various telecommunications stakeholders.

Military and security significance of the electromagnetic spectrum. Virtually all forms of military, espionage, and security activity exploit some portion of the electromagnetic spectrum. The transmission, reception, and interception of radio messages are perhaps the most obvious examples, second to the use of light in the visible spectrum for ordinary vision and most technical imaging. More exotic direct applications of electromagnetic radiation are also under development, including the direct use of electromagnetic waves (e.g., laser light) as a destructive weapon, and for various other methods of electronic warfare, defined by the U.S. Joint Chiefs of Staff as "any military action involving the use of electromagnetic and directed energy to control the electromagnetic spectrum or to attack the enemy." Jamming of enemy transmissions and protection of friendly forces against enemy jamming attempts are typical forms of electronic warfare.

In summary, it can be said that the manipulation of every level of the electromagnetic spectrum is of urgent technological interest, but most work is being done in the radio through the visible portions of the spectrum (below 7.5 × 1014 Hz), where communications, radar, and imaging can be accomplished.

FURTHER READING:

ELECTRONIC:

"Electromagnetic Spectrum Use in Joint Military Operations." Chairman of the Joint Chiefs of Staff Instruction. May 1, 2000. <http://www.dtic.mil/doctrine/jel/cjcsd/cjcsi/3320_01.pdf> (Jan. 30, 2003).

Schroeder, Norbert. "Radio Frequency Spectrum Allocations in the United States." National Telecommunications and Information Administration. July 1, 2000. <http://www.ntia.doc.gov/osmhome/chart_00.htm> (Jan. 30, 2003).

SEE ALSO

Electromagnetic Weapons, Biochemical Effects
Electronic Countermeasures
Electro-optical Intelligence

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Electromagnetic Spectrum

Electromagnetic Spectrum

The electromagnetic spectrum consists of all the frequencies at which electromagnetic waves can occur, ordered from zero to infinity. Radio waves, visible light, and x rays are examples of electromagnetic waves at different frequencies. Every part of the electromagnetic spectrum is exploited for some form of scientific or military activity; the entire spectrum is also key to science and industry. Forensic scientists often use ultraviolet light technologies to search for latent fingerprints and to examine articles of clothing. Infrared and near-infrared light technology is used by forensic scientists to record images on specialized film and in spectroscopy , a tool that determines the chemical structure of a molecule (such as DNA ) without damaging the molecule.

Electromagnetic waves have been known since the mid-nineteenth century, when their behavior was first described by the equations of Scottish physicist James Clerk Maxwell (18311879). Electromagnetic waves, according to Maxwell's equations, are generated whenever an electrical charge (e.g., an electron) is accelerated, that is, changes its direction of motion, its speed, or both. An electromagnetic wave is so named because it consists of an electric and a magnetic field propagating together through space. As the electric field varies with time, it renews the magnetic field; as the magnetic field varies, it renews the electric field. The two components of the wave, which always point at right angles both to each other and to their direction of motion, are thus mutually sustaining, and form a wave which moves forward through empty space indefinitely.

The rate at which energy is periodically exchanged between the electric and magnetic components of a given electromagnetic wave is the frequency, ν, of that wave and has units of cycles per second, or Hertz (Hz); the linear distance between the wave's peaks is termed its wavelength, λ, and has units of length (e.g., feet or meters). The speed at which a wave travels is the product of its wavelength and its frequency, V = νλ; in the case of electromagnetic waves, Maxwell's equations require that this velocity equal the speed of light, c (186,000 miles per second [300,000 km/sec]). Since the velocity of all electromagnetic waves is fixed, the wavelength λ of an electromagnetic wave always determines its frequency ν, or vice versa, by the relationship c = νλ The higher the frequency (i.e., the shorter the wavelength) of an electromagnetic wave, the higher in the spectrum it is said to be. Since a wave cannot have a frequency less than zero, the spectrum is bound by zero at its lower end. In theory, it has no upper limit.

All atoms and molecules at temperatures above absolute zero radiate electromagnetic waves at specific frequencies that are determined by the details of their internal structure. In quantum physics, this radiation must often be described as consisting of particles called photons rather than as waves; however, this article will restrict itself to the classical (continuous-wave) treatment of electromagnetic radiation, which is adequate for most technological purposes.

Not only do atoms and molecules radiate electromagnetic waves at certain frequencies, they can absorb them at the same frequencies. All material objects, therefore, are continuously absorbing and radiating electromagnetic waves having various frequencies, thus exchanging energy with other objects, near and far. This makes it possible to observe objects at a distance by detecting the electromagnetic waves that they radiate or reflect, or to affect them in various ways by beaming electromagnetic waves at them. These facts make the manipulation of electromagnetic waves at various frequencies (i.e., from various parts of the electromagnetic spectrum) fundamental to many fields of technology and science, including radio communication, radar, infrared sensing, visible-light imaging, lasers, x rays, astronomy, and more.

The spectrum has been divided up by physicists into a number of frequency ranges or bands denoted by convenient names. The points at which these bands begin and end do not correspond to shifts in the physics of electromagnetic radiation; rather, they reflect the importance of different frequency ranges for human purposes.

Radio waves are typically produced by time-varying electrical currents in relatively large objects (i.e., at least centimeters across). This category of electromagnetic waves extends from the lowest-frequency, longest-wavelength electromagnetic waves up into the gigahertz (GHz; billions of cycles per second) range. The radio frequency spectrum is divided into more than 450 non-overlapping frequency bands. These bands are exploited by different users and technologies: for example, broadcast FM is transmitted using frequencies on the order of 106 Hz, while television signals are transmitted using frequencies on the order of 108 Hz (about a hundred times higher). In general, higher-frequency signals can always be used to transmit lower-frequency information, but not the reverse; thus, a voice signal with a maximum frequency content of 20 kHz (kilohertz, thousands of Hertz) can, if desired, be transmitted on a signal centered in the Ghz range, but it is impossible to transmit a television signal over a broadcast FM station. Radio waves termed microwaves are used for high-speed communications links, heating food, radar, and electromagnetic weapons, that is, devices designed to irritate or injure people or to disable enemy devices. The microwave frequencies used for communications and radar are subdivided still further into frequency bands with special designations, such as "X band" and "Y band." Microwave radiation from the Big Bang, the cosmic explosion in which the Universe originated, pervades all of space.

Electromagnetic waves from approximately 1012 to 5 1014 Hz are termed infrared radiation. The word infrared means "below red," and is assigned to these waves because their frequencies are just below those of red light, the lowest-frequency light visible to human beings. Infrared radiation is typically produced by molecular vibrations and rotations (i.e., heat) and causes or accelerates such motions in the molecules of objects that absorb it; it is therefore perceived by the body through the increased warmth of skin exposed to it. Since all objects above absolute zero emit infrared radiation, electronic devices sensitive to infrared can form images even in the absence of visible light. Because of their ability to "see" at night, imaging devices that electronically create visible images from infrared light from are important in security systems, on the battlefield, and in observations of the Earth from space for both scientific and military purposes.

Visible light consists of elecromagnetic waves with frequencies in the 4.3 1014 to 7.5 1014 Hz range. Waves in this narrow band are typically produced by rearrangements (orbital shifts) in the outer electrons of atoms. Most of the energy in the sunlight that reaches the Earth's surface consists of electromagnetic waves in this narrow frequency range; our eyes have therefore evolved to be sensitive to this band of the electromagnetic spectrum. Photo-voltaic cellselectronic devices that turn incident electromagnetic radiation into electricityare also designed to work primarily in this band, and for the same reason. Because half the Earth is liberally illuminated by visible light at all times, this band of the spectrum, though narrow (less than an octave), is essential to thousands of applications, including all forms of natural and many forms of mechanical vision.

Ultraviolet light consists of electromagnetic waves with frequencies in the 7.5 1014 to 1016 Hz range. It is typically produced by rearrangements in the outer and intermediate electrons of atoms. Ultraviolet light is invisible, but can cause chemical changes in many substances: for living things, consequences of these chemical changes can include skin burns, blindness, or cancer. Ultraviolet light can also cause some substances to give off visible light (flouresce), a property useful for mineral detection, art-forgery detection, and other applications. Various industrial processes employ ultraviolet light, including photolithography, in which patterned chemical changes are produced rapidly over an entire film or surface by projecting patterned ultraviolet light onto it. Most ultraviolet light from the Sun is absorbed by a thin layer of ozone (O3) in the stratosphere, making the Earth's surface much more hospitable to life than it would be otherwise; some chemicals produced by human industry (e.g., chlorfluorocarbons) destroy ozone, threatening this protective layer.

Electromagnetic waves with frequencies from about 1016 to 1019 Hz are termed x rays. X rays are typically produced by rearrangements of electrons in the innermost orbitals of atoms. When absorbed, x rays are capable of ejecting electrons entirely from atoms and thus ionizing them (i.e., causing them to have a net positive electric charge). Ionization is destructive to living tissues because ions may abandon their original molecular bonds and form new ones, altering the structure of a DNA molecule or some other aspect of cell chemistry. However, x rays are useful in medical diagnosis and in security systems (e.g., airline luggage scanners) because they can pass entirely through many solid objects; both traditional contrast images of internal structure (often termed "x rays" for short) and modern computerized axial tomography images, which give much more information, depend on the penetrating power of x rays. X rays are produced in large quantities by nuclear explosions (as are electromagnetic waves at all other frequencies above the radio band), and have been proposed for use in a space-based ballistic-missile defense system.

All electromagnetic waves above about 1019 Hz are termed gamma rays (g rays), which are typically produced by rearrangements of particles in atomic nuclei. A nuclear explosion produces large quantities of gamma radiation, which is both directly and indirectly destructive of life. By interacting with the Earth's magnetic field, gamma rays from a high-altitude nuclear explosion can cause an intense pulse of radio waves termed an electromagnetic pulse (EMP). EMP may be powerful enough to burn out unprotected electronics on the ground over a wide area.

Radio waves present a unique regulatory problem, for only one broadcaster at a particular frequency can function in a given area. (Signals from overlapping same-frequency broadcasts would be received simultaneously by antennas, interfering with each other.) Throughout the world, therefore, governments regulate the radio portion of the electromagnetic spectrum, a process termed spectrum allocation. In the United States, since the passage of the Communications Act of 1934, the radio spectrum has been deemed a public resource. Individual private broadcasters are given licenses allowing them to use specific portions of this resource, that is, specific sub-bands of the radio spectrum. The United States Commerce Department's National Telecommunications and Information Administration (NTIA) and FCC (Federal Communications Commission) oversee the spectrum allocation process, which is subject to intense lobbying by various telecommunications stakeholders.

In summary, it can be said that the manipulation of every level of the electromagnetic spectrum is of urgent technological interest, but most work is being done in the radio through the visible portions of the spectrum (below 7.5 1014 Hz), where communications, radar, and imaging can be accomplished.

see also DNA fingerprint; DNA profiling; Electromagnetic weapons, biochemical effects; Fluorescence; Laser; Ultraviolet light analysis.

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Electromagnetic Spectrum

Electromagnetic spectrum

The electromagnetic spectrum encompasses a continuous range of frequencies or wavelengths of electromagnetic radiation, ranging from long wavelength, low energy radio waves, to short wavelength, high frequency, high-energy gamma rays. The electromagnetic spectrum is traditionally divided into regions of radio waves, microwaves, infrared radiation, visible light, ultraviolet rays , x rays, and gamma rays.

Scottish physicist James Clerk Maxwell's (18311879) development of a set of equations that accurately described electromagnetic phenomena allowed the mathematical and theoretical unification of electrical and magnetic phenomena. When Maxwell's calculated speed of light fit well with experimental determinations of the speed of light, Maxwell and other physicists realized that visible light should be a part of a broader electromagnetic spectrum containing forms of electromagnetic radiation that varied from visible light only in terms of wavelength and wave frequency. Frequency is defined as the number of wave cycles that pass a particular point per unit time, and is commonly measured in Hertz (cycles per second). Wavelength defines the distance between adjacent points of the electromagnetic wave that are in equal phase (e.g., wavecrests).

Exploration of the electromagnetic spectrum quickly resulted practical advances. German physicist Henrich Rudolph Hertz regarded Maxwell's equations as a path to a "kingdom" or "great domain" of electromagnetic waves. Based on this insight, in 1888, Hertz demonstrated the existence of radio waves. A decade later, Wilhelm Röentgen's discovery of high-energy electromagnetic radiation in the form of x rays quickly found practical medical use.

At the beginning of the twentieth century, German physicist, Maxwell Planck, proposed that atoms absorb or emit electromagnetic radiation only in certain bundles termed quanta. In his work on the photoelectric effect, German-born American physicist Albert Einstein used the term photon to describe these electromagnetic quanta. Planck determined that energy of light was proportional to its frequency (i.e., as the frequency of light increases, so does the energy of the light). Planck's constant, h =6.626×1034 joule-second in the meter-kilogram-second system, relates the energy of a photon to the frequency of the electromagnetic wave and allows a precise calculation of the energy of electromagnetic radiation in all portions of the electromagnetic spectrum.

Although electromagnetic radiation is now understood as having both photon (particle) and wave-like properties, descriptions of the electromagnetic spectrum generally utilize traditional wave-related terminology (i.e., frequency and wavelength).

Electromagnetic fields and photons exert forces that can excite electrons. As electrons transition between allowed orbitals, energy must be conserved. This conservation is achieved by the emission of photons when an electron moves from a higher potential orbital energy to a lower potential orbital energy. Accordingly, light is emitted only at certain frequencies characteristic of every atom and molecule. Correspondingly, atoms and molecules absorb only a limited range of frequencies and wavelengths of the electromagnetic spectrum, and reflect all the other frequencies and wavelengths of light. These reflected frequencies and wavelengths are often the actual observed light or colors associated with an object.

The region of the electromagnetic spectrum that contains light at frequencies and wavelengths that stimulate the rod and cones in the human eye is termed the visible region of the electromagnetic spectrum. Color is the association the eye makes with selected portions of that visible region (i.e., particular colors are associated with specific wavelengths of visible light). A nanometer (109 m) is the most common unit used for characterizing the wavelength of visible light. Using this unit, the visible portion of the electromagnetic spectrum is located between 380nm-750nm and the component color regions of the visible spectrum are: Red (670770 nm), Orange (592620 nm), Yellow (578592 nm), Green (500578 nm), Blue (464500 nm), Indigo (444464 nm), and Violet (400446 nm). Because the energy of electromagnetic radiation (i.e., the photon) is inversely proportional to the wavelength, red light (longest in wavelength) is the lowest in energy. As wavelengths contract toward the blue end of the visible region of the electromagnetic spectrum, the frequencies and energies of colors steadily increase.

Like colors in the visible spectrum, other regions in the electromagnetic spectrum have distinct and important components. Radio waves, with wavelengths that range from hundreds of meters to less than a centimeter, transmit radio and television signals. Within the radio band, FM radio waves have a shorter wavelength and higher frequency than AM radio waves. Still higher frequency radio waves with wavelengths of a few centimeters can be utilized for RADAR imaging.

Microwaves range from approximately a foot in length to the thickness of a piece of paper. The atoms in food placed in a microwave oven become agitated (heated) by exposure to microwave radiation. Infrared radiation comprises the region of the electromagnetic spectrum where the wavelength of light is measured region from one millimeter (in wavelength) down to 400 nm. Infrared waves are discernible to humans as thermal radiation (heat). Just above the visible spectrum in terms of higher energy, higher frequency and shorter wavelengths is the ultraviolet region of the spectrum with light ranging in wavelength from 400 to 10 billionths of a meter. Ultraviolet radiation is a common cause of sunburn even when visible light is obscured or blocked by clouds . X rays are a highly energetic region of electromagnetic radiation with wavelengths ranging from about ten billionths of a meter to 10 trillionths of a meter. The ability of x rays to penetrate skin and other substances renders them useful in both medical and industrial radiography. Gamma rays, the most energetic form of electromagnetic radiation, are comprised of light with wavelengths of less than about ten trillionths of a meter and include waves with wavelengths smaller than the radius of an atomic nucleus (1015 m). Gamma rays are generated by nuclear reactions (e.g., radioactive decay, nuclear explosions, etc.).

Cosmic rays are not a part of the electromagnetic spectrum. Cosmic rays are not a form of electromagnetic radiation, but are actually high-energy charged particles with energies similar to, or higher than, observed gamma electromagnetic radiation energies.

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Electromagnetic Spectrum

Electromagnetic spectrum

The term electromagnetic spectrum refers to all forms of energy transmitted by means of waves traveling at the speed of light. Visible light is a form of electromagnetic radiation, but the term also applies to cosmic rays, X rays, ultraviolet radiation, infrared radiation, radio waves, radar, and microwaves. These forms of electromagnetic radiation make up the electromagnetic spectrum much as the various colors of light make up the visible spectrum (the rainbow).

Wavelength and frequency

Any waveincluding an electromagnetic wavecan be described by two properties: its wavelength and frequency. The wavelength of a wave is the distance between two successive identical parts of the wave, as between two wave peaks or crests. The Greek letter lambda (λ) is often used to represent wavelength. Wavelength is measured in various units, depending on the kind of wave being discussed. For visible light, for example, wavelength is often expressed in nanometers (billionths of a meter); for radio waves, wavelengths are usually expressed in centimeters or meters.

Frequency is the rate at which waves pass a given point. The frequency of an X-ray beam, for example, might be expressed as 1018 hertz. The term hertz (abbreviation: Hz) is a measure of the number of waves that pass a given point per second of time. If you could watch the X-ray beam from some given position, you would see 1,000,000,000,000,000,000 (that is, 1018) wave crests pass you every second.

For every electromagnetic wave, the product of the wavelength and frequency equals a constant, the speed of light (c). In other words, λ · f = c. This equation shows that wavelength and frequency have a reciprocal relationship to each other. As one increases, the other must decrease. Gamma rays, for example, have very small wavelengths and very large frequencies. Radio waves, by contrast, have large wavelengths and very small frequencies.

Regions of the electromagnetic spectrum

As shown in the accompanying figure, the whole range of the electromagnetic spectrum can be divided up into various regions based on wavelength and frequency. Electromagnetic radiation with very short wavelengths and high frequencies fall into the cosmic ray/gamma ray/ultraviolet radiation region. At the other end of the spectrum are the long wavelength, low frequency forms of radiation: radio, radar, and microwaves. In the middle of the range is visible light.

Properties of waves in different regions of the spectrum are commonly described by different notation. Visible radiation is usually described by its wavelength, while X rays are described by their energy. All of these schemes are equivalent, however; they are just different ways of describing the same properties.

Words to Know

Electromagnetic radiation: Radiation that travels through a vacuum with the speed of light and that has properties of both an electric and magnetic wave.

Frequency: The number of waves that pass a given point in a given period of time.

Hertz: The unit of frequency; a measure of the number of waves that pass a given point per second of time.

Wavelength: The distance between two successive peaks or crests in a wave.

The boundaries between types of electromagnetic radiation are rather loose. Thus, a wave with a frequency of 8 × 1014 hertz could be described as a form of very deep violet visible light or as a form of ultraviolet radiation.

Applications

The various forms of electromagnetic radiation are used everywhere in the world around us. Radio waves are familiar to us because of their use in communications. The standard AM radio band includes radiation in the 540 to 1650 kilohertz (thousands of hertz) range. The FM band includes the 88 to 108 megahertz (millions of hertz) range. This region also includes shortwave radio transmissions and television broadcasts.

Microwaves are probably most familiar to people because of microwave ovens. In a microwave oven, food is heated when microwaves excite water molecules contained within foods (and the molecules' motion produces heat). In astronomy, emission of radiation at a wavelength of 8 inches (21 centimeters) has been used to identify neutral hydrogen throughout the galaxy. Radar is also included in this region.

The infrared region of the spectrum is best known to us because of the fact that heat is a form of infrared radiation. But the visible wavelength range is the range of frequencies with which we are most familiar. These are the wavelengths to which the human eye is sensitive and which most easily pass through Earth's atmosphere. This region is further broken down into the familiar colors of the rainbow, also known as the visible spectrum.

The ultraviolet range lies at wavelengths just short of the visible range. Most of the ultraviolet radiation reaching Earth in sunlight is absorbed in the upper atmosphere. Ozone, a form of oxygen, has the ability to trap ultraviolet radiation and prevent it from reaching Earth. This fact is important since ultraviolet radiation can cause a number of problems for both plants and animals. The depletion of the ozone layer during the 1970s and 1980s was a matter of some concern to scientists because of the increase in dangerous ultraviolet radiation reaching Earth.

We are most familiar with X rays because of their uses in medicine. X-radiation can pass through soft tissue in the body, allowing doctors to examine bones and teeth from the outside. Since X rays do not penetrate Earth's atmosphere, astronomers must place X-ray telescopes in space.

Gamma rays are the most energetic of all electromagnetic radiation, and we have little experience with them in everyday life. They are produced by nuclear processesduring radioactive decay (in which an element gives off energy by the disintegration of its nucleus) or in nuclear reactions in stars or in space.

[See also Frequency; Light; X rays ]

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electromagnetic spectrum

electromagnetic spectrum The range of wavelengths over which electromagnetic radiation extends. The longest waves (105–10–3 metres) are radio waves, the next longest (10–3–10–6 m) are infrared waves, then comes the narrow band (4–7 × 10–7 m) of visible light, followed by ultraviolet radiation (10–7–10–9 m), X-rays (10–9–10–11 m), and gamma rays (10–11–10–14 m).

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electromagnetic spectrum

electromagnetic spectrum The range of frequencies or wavelengths of electromagnetic radiation.

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