The Physics of War (29 page)

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.
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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 Han
s 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:

• To detect the location of an object that is at some distance away and cannot be observed visually.
  
• To detect the speed of the object.
  
• To generate a topological mapping of an area of the earth.
  

Radar is accomplished using an echo as well as what is called the Doppler Effect.
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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.

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.
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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.

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.