images

Electromagnetic radiation has played a large role in warfare since World War I, especially as used in radar, radio, and lasers. To understand these technologies, however, we have to go back several years before the beginning of World War I.

THE PRODUCTION AND DETECTION OF ELECTROMAGNETIC WAVES

James Clerk Maxwell is regarded by many as one of the most important physicists ever born. His prediction of the existence of electromagnetic waves led to major advances in science and also to important changes in everyday life.1

In the mid-1800s, four basic things were known about electricity and magnetism:

These facts were known before Maxwell's time. His contribution was to put them in mathematical form and show that electricity and magnetism are intimately related, together forming what we call an electromagnetic field. In particular, oscillating electric charges produce an electromagnetic field that moves out from the charges, and the waves produced have both an electric and a magnetic field associated with them. Of particular importance, Maxwell found that electromagnetic waves traveled at the speed of light, and he proposed that light itself was an electromagnetic wave. Furthermore, he pointed out that electromagnetic waves of higher and lower frequencies (frequency at which the charge was oscillating) would likely lie beyond the frequency of light. In other words, there should be an array of electromagnetic waves of all different frequencies. And indeed, we now know that this is the case.

His prediction was made in the 1860s, and it wasn't long before electromagnetic waves were detected directly. In August 1879 the German physicist Heinrich Hertz built a simple device in his lab that he believed could be used to detect Maxwell's waves. It consisted of a loop of wire with a gap to which small brass knobs had been attached. The loop was connected to an induction coil so that a spark could be generated across a gap. He then built a second loop with an induction coil to act as a detector. When the first loop was connected to the induction coil, a spark jumped across the gap, sending out a “signal.” This signal was detected by the second loop (the receiver), which was nearby. Hertz was able to show that the signal exhibited a wave nature, and that it had a certain wavelength, or frequency, so it was obviously another electromagnetic wave. Furthermore, he was able to calculate its speed, showing that it was equal to the speed of light. He announced his discovery in 1887, claiming it was a verification of Maxwell's prediction.

images

Hertz's apparatus for detecting electromagnetic waves.

THE ELECTROMAGNETIC SPECTRUM

Maxwell was right: there was, indeed, a large spectrum of electromagnetic waves. We now know that they range from very short wavelength (high frequency) gamma rays down to very long wavelength (low frequency) radio waves. In between these two extremes are x-rays, ultraviolet (UV) light, visible light, infrared radiation and microwaves. Furthermore, all of these waves carry energy, or, more exactly, they are a form of energy, and the magnitude of their energy depends on their frequency (number of vibrations per second).2

images

The electromagnetic spectrum.

Several of these types of radiation had already been detected. In 1800 the German-born astronomer William Herschel was studying the temperatures of different colors by moving a thermometer along the spectrum of colors created by a prism when he noticed that the highest temperature was actually beyond red, which was at the end of the spectrum. He concluded that sunlight contained a heat-type radiation that could not be seen. We now know that this is infrared radiation. You can easily detect it when you turn on the burner of an electric stove. You feel the heat long before the burner turns red.

The following year Johann Ritter was looking at the other end of the visible part of the spectrum when he detected invisible rays that were similar to violet rays, but beyond them in the spectrum. He called them “chemical rays,” but their name was later changed to ultraviolet rays.

Also, years earlier, in 1895, Wilhelm Röntgen of Germany had noticed a type of high-energy radiation that was created when an evacuated tube was subjected to high voltage. He called the waves x-rays. And Hertz, in some of his earlier experiments, had already discovered microwaves and radio waves. Finally, in 1910 the British physicist William Bragg showed that there were very energetic waves with energies even higher than those of x-rays. These are called gamma rays.

Let's go back now and look more carefully at how we identify each of these types of radiation. As we just saw, they differ in their vibration rate, or frequency. And since frequency is related to wavelength (the distance between equal points along the wave), they also differ in wavelength. In addition, they differ in how energetic they are (we will talk more about this later). For the most part we will identify them by their frequency. The unit of measure for frequency is the Hertz (Hz), which is the number of vibrations per second. The range in frequency, however, is so large that we sometimes have to use units such as megahertz (MHz), which is one million Hertz. Infrared light, for example, has a frequency of up to approximately 100,000 MHz. Microwaves, which you are no doubt familiar with in relation to microwave ovens, have a range from 1,000 to 100,000 MHz. And radio waves go from 1,000 MHz down to 50 MHz. For radiation of even higher frequency we have to use gigahertz (GHz), which is 1 billion Hertz. Infrared radiation is in this range. And finally, beyond infrared, through visible and UV light, a unit called the Terahertz (THz), or 1 trillion Hertz, is used.

images

Diagram of a wave showing wavelength and amplitude.

Almost all of these types of radiation have important applications to war. Radio waves are used extensively for communication, and, as we will see, radar played a critical role in World War II, and it is still used extensively. We will also discuss lasers; most lasers use visible light, but other radiation frequencies are now used to produce other types of lasers, and lasers have played an important role in the military. Infrared radiation also has an important application to the military in relation to various devices that are used for night vision. And finally, x-rays are, of course, critical in the treatment of wounded soldiers.

RADIO WAVES

Radio waves are one form of electromagnetic radiation, and it wasn't long after Hertz's discovery that scientists began experimenting with them. One of the first to do so was Guglielmo Marconi (1874–1937) of Italy. When Hertz died in 1894 there was a sudden renewed interest in his discoveries and many newspapers published articles on them. Hertz's work came to the attention of Marconi, who was only twenty years old at the time. He was sure the waves Hertz had discovered could be used to create a system of wireless telegraphy, in other words, a telegraphic system that could send messages without using wires. As a result, he set up a simple system to see if this was feasible. His system consisted of a simple oscillator (a spark-producing radio transmitter) and a “coherer” receiver, which was a modification of an earlier receiving device. He used a telegraphic key to operate the transmitter so that it would send a series of long and short pulses (dots and dashes); a telegraphic receiver was activated by the coherer.3

By the summer of 1895 he was able to transmit and receive messages over a distance of a mile and a half. He decided at that point that he would need funding to improve the device. Finding little interest in Italy, he traveled with his mother to England, where he demonstrated his device to William Preece, the chief electrical engineer of the British General Post Office. A series of demonstrations to government officials followed, and with their support, in March 1897, Marconi was able to send a message over a distance of 3.7 miles.

Marconi and his experiments began to attract international attention. In 1899 he set up equipment on two sides of the English Channel and sent a message from France to England. Shortly thereafter he sailed to the United States at the invitation of the New York Herald. In the following year he began working on equipment to send a message across the Atlantic, and on December 12, 1901, he claimed that he had accomplished his goal. There was, however, some skepticism, so in February 1902, he set up a more advanced apparatus and proved the skeptics wrong. There was no doubt that he had accomplished his goal.

One of the early problems for long-distance radio transmission, or at least anticipated problems, was the curvature of the earth. Since radio waves travel in straight lines, it was expected that this curvature would cut off the signal. Marconi was pleased to find this didn't happen. The reason was that the radio waves were reflected, or bounced, back and forth due to the presence of electrically charged particles in the atmosphere.

Marconi continued to work on his device over the years, but he soon discovered he had competition. His messages used a series of dots and dashes (Morse code), but in the early 1900s, the first vacuum tube was invented, and as a result, wireless voice transmission became possible. This new development quickly overshadowed telegraphic transmission.

Of particular interest, however, was that war departments on both sides of the Atlantic were soon interested in Marconi's device. The British War Office was one of Marconi's first customers, and soon after major German telegraphic firms began buying his products; he set up a company in Germany to start selling them around 1900.

Radio soon started to play an important role in war. Over the next few years transmitters and receivers were improved significantly and radio became the major communication media during war. It was starting to be used by both sides in World War I, and it was, of course, used extensively in World War II.

X-RAYS

Another type of electromagnetic radiation critical in war is x-rays, and its use is mainly for saving lives rather than for ending them. The beginning of x-ray technology goes back even further than that of radio technology. X-rays were discovered by the German physicist Wilhelm Röntgen.4 At the time, the scientific world was intrigued by a discovery that had been made a few years earlier. High-voltage electrical currents in an enclosed tube containing rarified gases created what were called cathode rays. They had attracted a lot of attention, and Röntgen began experimenting with them. On the evening of November 8, 1895, he discovered something strange. He was particularly interested in a glow, called luminescence, which occurred in certain chemicals, and he wanted to find out if cathode rays would cause luminescence. While working in a darkened room, he noticed that a sheet of paper that he had covered with platinocyanide was glowing. This was strange because the cathode rays were not striking it directly; in fact, they were blocked off from it, yet when he turned the cathode-ray tube off, the glow disappeared. It soon became obvious to him that some type of radiation was being emitted by the cathode-ray tube, but it was invisible, and he discovered that it was coming from the point where the cathode rays were hitting the glass. Checking it out further, he found that this new radiation was highly penetrating. Not only did it pass through wood and thin sheets of metal; it also passed completely through his hand. Furthermore, a photograph using it showed the bones of his hand. He knew immediately that this new type of radiation would have important medical applications, particularly in relation to broken bones, and perhaps in locating bullets and so on within a person's body. He was unsure what to call the waves, so he referred to them as x-rays, and the name stuck. Within a short time (1900) he was awarded the first Nobel Prize for his discovery.

X-rays have, indeed, become an important tool in relation to war. During World War I, x-ray equipment became a major component of many first-aid stations and hospitals near the field of action. Madam Curie was one of the first to encourage the use of x-rays for treating wounded soldiers in World War I. Over the years, x-ray equipment has improved significantly, and it is now a major tool in war.

LIGHT AND INFRARED

It might seem strange that ordinary light is an electromagnetic wave, but indeed it is. This means that x-rays and light are basically the same; the only difference between them is their frequency. And, as we have seen, frequency is directly related to energy. The frequency of x-rays is much higher than that of light, so x-rays are much more energetic. That's why they easily penetrate your body and can be dangerous.

We can also say that ordinary light is an important weapon of war because telescopes and binoculars have played an important role in war ever since magnifying lenses were invented. The first practical telescope was invented by the Dutch optician Hans Lippershey in 1604, but he kept it secret for several years. Five years later, however, Galileo heard of the discovery and built a telescope for himself.

Simple telescopes have two main lenses: a relatively large convex lens (curved outwardly on both sides) called an objective, and a smaller lens called an eyepiece. Such devices are known as refracting telescopes. A different type of telescope, called a reflecting telescope, uses a mirror rather than a large convex lens. The reflecting telescope was invented by Newton, and it is used mainly in astronomy. Refracting telescopes were used extensively in early wars, and they are still used today. Hans Lippershey also built a binocular version of his telescope in 1608, with two telescopes mounted side-by-side, but it was quite crude. Box-shaped binocular telescopes for terrestrial use were produced in the second half of the seventeenth century and the first half of the eighteenth century by several people, but they were still rather crude.

Modern binoculars use a system of prisms that was discovered in 1854 by Ignazio Porro of Italy. Other systems are also used. Several lenses, in addition to the objective and eyepiece, are now used in modern binoculars.

Turning now to infrared radiation, we find that one of the most useful military applications is the use of infrared for enhanced night vision. Special infrared goggles allow a person to see much better at night. Two types of devices are used. The first uses the infrared wavelengths closest to visible light for image enhancement, using a special tube, called the image-intensifier tube, to collect and amplify the infrared light in this region. (It also gathers some visible light.) A conventional lens captures this light and sends it to the image-intensifier tube, which converts the light signal to electrons with the same distribution. An electron multiplier then increases the beam, keeping the same distribution pattern. These electrons then hit a screen coated with phosphor, which causes them to release a light pattern with the same image as the original one, but enhanced.

The second device uses thermal imaging. It focuses on the infrared region most distant from visible light. In this case a temperature pattern called a thermogram is created. This thermogram is then converted to electrical impulses, and these impulses are sent to a unit called the signal-processing unit, which translates them into a form suitable for display.

Night-vision lenses such as the above are used extensively by the military to locate targets at night. They are also used for surveillance and for navigation.

Lasers, another important recent discovery, also use light in this region, but they will be discussed in a later chapter.

RADAR

Radar is another technology that uses electromagnetic radiation, and, as we'll see in chapter 16, it played a large role in World War II, and it has played a significant role in the military ever since. The word radar is actually an acronym for Radio Detection and Ranging. For the most part, it is used for one or more of the following:

Radar is accomplished using an echo as well as what is called the Doppler Effect.5 While most people are familiar with echoes, the Doppler Effect is less well known, so I'll explain it. Although radar generally uses microwaves, it is easiest to understand the concepts by explaining them in terms of sound waves. The relevant phenomena are essentially the same. And sound waves are in fact used in the same way as microwaves for what is called sonar. Sonar is important in relation to submarines, and we will discuss it in detail in the next chapter.

Now back to the Doppler Effect. As you no doubt know, sound waves have a certain wavelength, or frequency, in the same way electromagnetic waves do. With this in mind, consider a car approaching you with its horn blaring. The sound wave is traveling away from the car at the speed of sound, but the car is moving, so it is “catching up” with the sound wave. Because of this, the sound wave gets slightly compressed, and this means that its wavelength is shortened (see figure). When the car passes, however, you experience an opposite effect because the two velocities are now in opposite directions. In this case the wavelength is lengthened because the wave is stretched out. As a result, you notice that when the car is approaching, the horn's pitch, or frequency, is higher than it would be if the car were sitting still, and it is lower once the car passes you. The effect was discovered by the Austrian physicist Christian Doppler (1803–1853). It not only occurs with sound waves, but all waves, including radio waves.

images

The Doppler Effect. Sound waves are squeezed together in the direction of travel and separated in the opposite direction.

It's easy to understand how we can determine the distance to an object using an echo. If you know the speed of sound in air (it is approximately 1,126 feet per second), and if you measure the time it takes for sound to reach an object and bounce back, the distance of the object can be determined by dividing the time you measured by two and then multiplying the result by the velocity of sound. So the echo gives you the distance. But you can also combine the time of the echo with the Doppler Effect to calculate the speed of an object, such as the car in the previous example. In this case, assume you send a sound signal toward a car that is approaching you. Some of the sound waves will bounce off the car and eventually create an echo, but most will scatter off in other directions. The ones that scatter off in other directions can be ignored. So we will have an echo, but at the same time, since the car is moving toward us, the sound waves will be compressed. The waves of the returning echo will therefore have a higher pitch compared to the original ones. If you measure the difference in pitch of the returning waves, as compared to the ones you sent out, you will be able to determine how fast the car is going. And because the time of the echo gives you the distance, you have both the car's speed and its distance.

In practice, however, sound waves don't work very well. First of all, in most cases the echo would be hard to detect and measure. It would be faint and there would be a lot of interference. Furthermore, sound doesn't travel very far before it decays away. Microwaves don't have this problem, however, and this is why they are used in radar.

So let's set up a simple radar system using microwaves and look at it. Assume that we want to use it to detect enemy planes that are concealed by fog or clouds. First we need to send out a microwave signal. The best form for this signal, as we will see, is a short burst of waves. Assume the burst is one microsecond long (a millionth of a second); in other words, we only turn on the transmitter for one microsecond. This burst will leave the transmitter and travel toward the target; when it reaches it, it will strike the target, and most of it will be reflected. Most of it, in fact, will be reflected in random directions, but some will come back directly to the transmitter, and the pulse that does can be detected and amplified. For this, of course, we will need a receiver, and this receiver is usually (but not always) at the same location as the transmitter. So as soon as the radar transmitter sends out its signal, it is turned off, and the receiver is turned on to listen for the echo. Since radar waves travel at the speed of light, it doesn't take long for the echo to reach the receiver. Immediately after it is picked up, electronic devices measure the time it took for its flight, and they also measure the shift in its wavelength, in other words, its Doppler shift. This information is sent to a computer in the unit that calculates the distance and speed of the approaching airplane, or whatever was being detected.6

A radar system can actually detect more than just the speed and distance of an enemy plane. It can also detect the plane's altitude and the direction in which it is flying. And airplanes aren't its only targets. It can be used to detect ships at sea, spacecraft, guided missiles from another country (being shot at us), weather formations such as storms and other phenomena, and the topography of terrain. And it doesn't matter if the targets are obscured by clouds or most other atmospheric phenomena. So radar is obviously a vital part of any defense system.

Let's look at the device in more detail, starting with the transmitter. It sends out microwave radar signals in the direction of the target. Radar signals are reflected by most metals and by carbon fibers; this is why radar is ideal for the detection of airplanes, ships, cars, missiles, and so on. However, radar signals are not reflected well from radar-absorbing materials such as high-resistant materials and certain magnetic materials, and because of this, these materials are used for various types of military crafts and vehicles so that they can avoid radar.

images

A simple radar system.

And there are also problems in relation to receivers. The reflected microwave beam is usually very weak, so the receiver has to amplify it. Radar beams are, in fact, scattered off a target in the same way that light is scattered off mirrors, but there is an important difference. Ordinary light has a very short wavelength, but microwaves have a relatively long wavelength. And if the radar receiver is to “see” the target properly, the wavelength of the radar signal has to be much shorter than the target size. Early radars used relatively long wavelength signals (in the radio region) and therefore had difficulty in interpreting the reflected signal. More modern radar units, however, now use the relatively short wavelengths of microwaves.

Another problem with radar is that microwaves in the atmosphere and even within the device itself can interfere with the signal. This interference superimposes itself on the radar signal and has to be reduced or “cleaned up” before the returning signal can be analyzed properly. The interfering waves may come from buildings, mountains, and other objects that reflect microwaves.

AN AMAZING DISCOVERY

In the late 1930s it became obvious that Germany was building up its military and would likely unleash an all-out attack on Britain in the near future. And it was also known that the Germans had close to three thousand planes compared to only eight hundred for Britain. As a result, the British set up an extensive system of radar stations, but radar still had serious problems at that time. It was low-powered and used radio waves that did not give a clear image. The British needed something better, and they needed it fast. The shortest wavelengths available were about 150 centimeters (59 inches) with the power of about 10 watts.7

Scientist began a search. It was soon noticed that a General Electric physicist, Albert Hall, at Schenectady, New York, had invented a simple device he called a magnetron in 1920. It looked like it had promise, but he couldn't think of any uses for the device at the time. Hall's device did not generate microwaves, but it was soon discovered that with a slight modification, it might be able to generate them, and, as a result, it attracted some attention. It wasn't until the late 1930s, however, when two engineers in England, Harry Boot and John Randall, decided to explore the device further that people got really excited. Hall's earlier device consisted of a cathode (negative terminal) and an anode (positive terminal) in a glass tube, quite similar to an ordinary vacuum tube. Boot and Randall modified it; they used a copper body, which acted like an anode. It was cylindrical with several cylindrical cavities around its inner edge. These cavities opened into a central vacuum chamber that contained the anode. A permanent magnet was used to create a magnetic field that ran parallel to the axis of the cylinder. The cathode was hooked to a high-voltage power supply. This produced electrons that streamed out toward the cylinder walls. These electrons, however, were deflected by the magnetic field into curved paths, and this caused them to set up small circular currents within the cavities. These currents produced microwave radiation that could be directed into a device called the waveguide, which channeled it to an outside device where it could be used. Of particular interest, the wavelength of the microwave radiation was related to the size of the cavity, and therefore it could be adjusted.

When Boot and Randall completed their device in February 1940, they tested it and were amazed that it produced microwaves with the power of nearly five hundred watts—fifty times what the earlier devices were capable of. Furthermore, the wavelength of the microwave radiation was only 10 centimeters (3.93 inches), which would give a much clearer picture of enemy objects. In addition, the device was small enough to fit into the palm of your hand. They were delighted, and over the next few months they worked to perfect their device.

images

The cavity magnetron.

By now, however, the war had started, and Britain was strapped for money. But the British needed the device; in fact, they needed a large number of them for their radar-defense systems against German planes. Churchill knew that Britain could not produce the large numbers needed, but the United States could, and he also knew that the United States was working on its own radar system and would be amazed at the device that Boot and Randall had devised. He therefore suggested that Henry Tizard, the chairman of the Aeronautical Research Committee, offer the magnetron, as it was called, to the United States in exchange for help in mass-producing it.

In a secret mission that took place in September 1940, Tizard went to United States. In a small box he carried a magnetron capable of generating 500 watts (while the most powerful magnetron in United States at the time could create only about 10 watts). And indeed, within a short time, a deal was reached. American officials later described the device as “the most valuable cargo ever brought to our shores.”8

Scientists at Bell Labs made a copy of the device suitable for mass production before the end of 1940, and a lab was set up at MIT (Massachusetts Institute of Technology) to develop a more powerful radar system using it. Back in England, scientist at TRE (Telecommunications Research Establishment) developed a revolutionary new radar system that could be used by airplanes for ground mapping.

The magnetron, which is usually called a cavity magnetron because of the small cavities within it, allowed the detection of very small objects such as submarine periscopes. And since magnetrons were now small enough to be installed in airplanes, a squadron of airplanes could easily spot enemy subs and destroy them. The new device also proved valuable in detecting incoming German bombers well before they got to England, so the Royal Air Force could prepare for them. And it also improved the accuracy of Allied bombing raids over Germany. This will be discussed in much more detail in chapter 16.