The same thing is true of electromagnetic radiation. Our eyes, like an ocean liner, sense a very narrow range of wavelengths—those around a few thousand atom diameters (about a ten thousandth of an inch)—but longer and shorter waves are all around us. The complete set of these waves is called the electromagnetic spectrum. All types of waves in this spectrum travel at 186,000 miles per second, and all are produced by moving electromagnetic fields.
With his discovery Maxwell not only solved the mystery of the nature of light, but pointed to extraordinary practical consequences. As soon as he realized that visible light is only a narrow band of electromagnetic radiation, he postulated the existence of other waves of both longer and shorter wavelengths. These other waves included what we now call radio, microwave, infrared, ultraviolet, X-rays, and gamma rays.
There is no theoretical limit to the wavelength of electromagnetic radiation; frequencies from zero to infinity are possible. In practice, however, we can only detect a limited range of waves, from radio waves a few thousand miles long to gamma rays with wavelengths smaller than atomic nucleii. Scientists and engineers have divided the spectrum into several regions, somewhat arbitrarily, based on how the radiation is produced and how it is detected.
Radio waves, microwaves, visible light, and X-rays are all parts of the electromagnetic spectrum. Electromagnetic waves, which surround us all the time, are produced any time an electric charge accelerates. Frequencies range from thousands (kilohertz) or millions (megahertz) of cycles per second to many trillions of cycles per second.
Radio Waves
Radio waves encompass all electromagnetic radiation with wavelengths of a few yards to thousands of miles, the longest waves that we can easily produce and detect. Radio waves are very useful because they travel through air without being absorbed, they are easily generated and detected, and the longer wavelengths bend around Earth’s curvature. Radio waves are the ideal medium for global communication. When you watch TV or listen to your car radio, you are using signals that have been transmitted by radio waves.
Both radio and television signals begin in tall antennas, where electrons are accelerated back and forth to create electromagnetic waves. All stations have a basic “carrier” frequency—the frequency of the wave that you read on the radio dial. The way that music or conversation is impressed on the carrier depends on the type of signal being sent. FM stations vary the frequency slightly (frequency modulation) while AM stations vary the signal strength (amplitude modulation). The difference between AM and FM is analogous to sending signals with a flashlight. If you send a signal by alternately dimming and brightening the flashlight, you are acting like an AM station. If, on the other hand, you send a signal by changing the color of the emitted light, you are like an FM station.
AM radio waves are about 1,000 feet in wavelength—long enough to bend around Earth’s curvature. A strong station can be heard for hundreds of miles, especially at night when interference from other electromagnetic radiation is minimal. FM stations use radio waves only a few feet in wavelength. These waves do not bend around Earth, so FM stations must rely on line-of-sight transmission. This is why your favorite FM stations fade out when you drive more than about 50 miles from town.
The radio part of the electromagnetic spectrum is wide, but it can only accommodate a finite number of separate channels. In addition to thousands of radio and television stations, there are hundreds of thousands of marine, aviation, amateur, and public safety broadcasters. The vast number of radio transmitters now in use would hopelessly clutter the airways, leading to electromagnetic chaos, without strict international controls over the allocation of broadcast frequencies, licensing, and operation of all radio stations. One of the principal responsibilities of the International Telecommunications Union and regional groups like the U.S. Federal Communications Commission is to allocate the long-wavelength end of the electromagnetic spectrum so that no two stations have frequencies that overlap.
Microwaves
Microwaves are electromagnetic waves about a tenth of an inch to a foot long. Longer microwaves, which have many features in common with radio waves, pass freely through air and can carry information. Unlike radio transmission, however, microwaves can be focused into a beam and therefore are often sent in highly directional signals, relayed with security from one cluster of hornlike antennas to another across the countryside. Furthermore, micro waves can be fine-tuned to yield a hundred times more useful frequencies than radio.
Line-of-sight transmission is essential for microwaves, so many microwave transmitters are prominently situated on tall towers or hilltops. More recently, microwaves have been used to communicate between Earth’s surface and satellites, which then beam the signal back to a different point on the earth. Many of the long-distance phone calls made in the United States are now routed through satellites via microwaves, as is satellite television. The TV dishes you see in backyards and on rooftops and in hotel parking lots are all designed and carefully positioned to receive micro wave signals sent down from satellites in fixed orbit. Commercial cellular phones operate in the same way, providing a link between a central transmitter and the mobile phone. In order to avoid cluttering the available channels, electronic systems break something like a large city into small units (“cells”), each with its own channel, and pass you along from one cell to the next, using whatever channel is available in each new cell.
Since World War II, microwaves have played a vital role in aircraft tracking. Radar employs directional microwave pulses, which reflect off solid objects in the air. The most sophisticated modern radar can pinpoint the location of a housefly at a distance of more than a mile.
Your microwave oven makes a very different use of electromagnetic radiation. The heart of the oven is a magnetron, a vacuum tube in which electrons can move. A beam of electrons in the magnetron oscillates about a billion times a second to produce microwaves about a foot in wavelength. In your food, this particular radiation is absorbed by water molecules (clusters of two hydrogen atoms and one oxygen), which are then set into violent vibrations as the energy of the radiation is converted into molecular energy of motion. This molecular motion makes the food hot.
Infrared Radiation
The infrared portion of the electromagnetic spectrum extends from wavelengths of about a hundred thousandth of an inch to a tenth of an inch. The long wavelength end overlaps with microwaves, while the short wavelength end stops at visible red light. Every warm object gives off infrared radiation. In the classic cowboy movie, for example, the scout who holds his hands toward the remains of a campfire and announces that the bad guys are only an hour away is sensing infrared radiation emitted by the still cooling embers.
The infrared radiation we most commonly experience originates from vibrations of molecules. When you sit in front of a fire the molecules of burning wood vibrate wildly, releasing heat radiation. That energy travels at the speed of light and is absorbed by your skin, setting your own molecules into vibration and triggering nerve impulses—you feel the heat.
Infrared radiation is absorbed in the atmosphere, so it is not very useful for long-distance communications. It is, however, widely used in devices like remote controls for TV sets and other situations where signals have to travel only a short distance. Even though our eyes can’t see it, all objects absorb and emit infrared radiation. Each type of material has its own distinctive infrared “color,” so many nocturnal animals have developed infrared vision. Special infrared cameras in orbit around the Earth take advantage of the same phenomenon, as do night vision systems that convert infrared radiation to visible images. These devices are widely used in the military and are now commonly used as night driving aids in automobiles as well.
Visible Light
Visible light is the narrowest, but the most obvious, of the spectral regions. Most human eyes can detect waves between about 16 and 32 millionths of an inch long—roughly the distance across 5,000 atoms. Light is further divided into a spectrum of
colors: red, orange, yellow, green, blue, indigo, and violet—literally the colors of the rainbow. Of these colors, violet has the shortest wavelength (and therefore the highest frequency and energy), while red has the longest wavelength.
The importance of light to us sometimes makes it difficult to keep in mind its relative insignificance in the grand sweep of the electromagnetic spectrum.
Ultraviolet Radiation
Ultraviolet light starts at wavelengths just shorter than visible violet. This so-called black light is used in a wide range of applications. Many U.S. postage stamps are tagged with fluorescent ink, theatrical productions incorporate colorful fluorescent paints, and amusement parks employ fluorescent hand stamps so visitors can come and go.
Shorter wavelength ultraviolet radiation has enough energy to disrupt and kill cells. Electromagnetic waves less than about a millionth of an inch in length are readily absorbed by living things and deposit sufficient energy to split apart molecules. For this reason, these wavelengths are routinely used in hospitals to sterilize equipment.
Ultraviolet radiation is absorbed in the atmosphere, particularly by ozone gas. The energetic radiation that leaks through this shield causes sunburn or even cancer on exposed skin. With UV radiation we begin to enter the dangerous region of the electromagnetic spectrum.
X-rays
Electromagnetic radiation with wavelengths about the size of an atom (a ten millionth of an inch) are called X-rays. Their accidental discovery in 1895 revolutionized diagnostic medicine, since it gave physicians a chance to examine the interior of the human body without surgery.
X-ray machines at your doctor’s or dentist’s office are usually bulky metal things of odd dimensions, painted some depressing shade of green or gray. The workings are well concealed, but always contain two basic components that are housed in a high vacuum. At one end is a thin wire filament, similar to the one found in an ordinary incandescent lightbulb. When heated to thousands of degrees, the filament emits a steady torrent of electrons, pulled out by strong electrical forces. The electrons are then accelerated toward a positively charged metal plate. They smash into the metal, and their deceleration unleashes a flood of energetic electromagnetic radiation—X-rays.
The fact that X-rays can go through solid matter makes them useful not only in medicine, but in the study of materials as well. By studying how this sort of radiation interacts with a crystal, for example, scientists can deduce how the atoms inside the crystal are arranged.
Gamma Rays
Gamma rays, the most energetic electromagnetic radiation that we can measure, are produced in stars (cosmic rays) and during some radioactive decay. They have wavelengths much smaller than individual atoms and are therefore capable of passing through most solids. Because of their high energy, gamma rays are routinely used to treat tumors by destroying the cancerous cells. They also figure in a number of advanced medical testing procedures.
FRONTIERS
The Wireless World
Because electricity and magnetism have been studied intensively for a century and a half, little is being done in the way of basic scientific research in these fields today. However, this neglect does not mean that the field is dormant—far from it. Remarkable technological advances are leading to major changes in the life of people in industrialized countries, as we find new ways to use the electromagnetic spectrum to communicate.
Up until the nineteenth century, communication between people could take place only face-to-face or by written messages—what we refer to now as “snail mail.” In the 1800s, two new inventions—the telegraph and the telephone—changed that. For the first time, people could communicate in real time over large distances. This sort of communication, however, still required a physical connection, a real copper wire, between the sender and receiver. In the 1980s even ARPANET, the prototype of today’s Internet, required that computers be connected to each other with high-speed telephone lines.
In a sense, today’s wireless technology goes back to Marconi and that first radio transmission. The difference is that modern computer technology (see Chapter 7) allows us to send much more information in a shorter time than Marconi could ever have dreamed of. Typically, a modern Wi-Fi system (the term is short for “wireless fidelity”) connects to a computer network by sending and receiving radio waves. As outlined above, short-range wireless communication (between parts of a computer system in the same room, for example) uses infrared radiation. Systems that require wide availability may make use of micro waves sent down from satellites. Researchers are already hard at work expanding wireless capability (think of the advances in cell phones over the past few years, for example), and there is even interest in finding ways of sending electrical power through wireless channels, so that you would never have to plug in your laptop or phone.
It is already hard for many people to remember what the world was like before the Internet. Imagining what the wireless world will be like twenty years from now is almost impossible.
CHAPTER FOUR
The Atom
WHAT DO THE FOLLOWING things have in common?
an elephant
panty hose
the Empire State Building
sand
your left ear
the Pacific Ocean
air
tofu
Jupiter
beer
this book
The answer is simple:
All matter is made of atoms.
Every tangible thing—the book you read, the food you eat, the air you breathe—is made of atoms. Atoms are the building blocks of matter. The atoms in turn are made largely from three types of smaller particles: protons and neutrons in the atomic nucleus, and electrons that orbit the nucleus. All of the amazing diversity of atoms—chemical elements as different as hydrogen, copper, sulfur, and uranium—results from different combinations of these three subatomic particles.
HOW DO WE KNOW THEY’RE THERE?
Atoms are a physical reality, but not one that you can verify just by looking around you. Atoms are so small that a million atoms placed end to end are no longer than a period on this page. The head of an ordinary pin contains more than 1,000,000,000,000,000,000 atoms. But although each atom has a minuscule mass, Nature has more than made up for the insignificance of each atom by producing a vast number of them.
The original idea of the atom is usually associated with the Greek philosopher Democritus, who lived sometime in the fifth century B.C. His argument went something like this: Imagine that you have a very sharp knife and a piece of cheese. Cut a piece off the cheese, then cut that piece, then cut the resulting smaller piece, and so on and on. Two things might happen: you will either come to a smallest piece of cheese—the cheese atom—or you won’t. Either possibility is reasonable. After all, you can build a house with individual bricks or from poured concrete. At a distance you see the house, but you can’t tell how it is built. On philosophical grounds, Democritus argued that smallest bits must exist, and he gave them the name “atom”—that which cannot be divided. It wasn’t until the early nineteenth century that the English chemist John Dalton (1766–1844) put forward our modern notion of the atom. Dalton was driven to believe in atoms by the results of laboratory experiments. Researchers discovered that most substances they encountered could be broken down in one way or another—by burning, by immersion in acid, or some other procedure. Occasionally, however, they would run across something that could not be broken down at all. Dalton called these substances, including oxygen, gold, sulfur, and iron, “elements.”
Many common chemicals consist of precise ratios of elements. Water, whether taken from Arctic ice or tropical rain or distilled from living things, always has an exact ratio of hydrogen to oxygen of 1:8 by weight. Dalton guessed that each chemical element is represented by its own atom, and these atoms combine in simple ways. Water, for example, is made from two atoms of hydrogen and one of oxygen.
Throughout the nineteenth and early twentieth centur
y, a debate went on over whether atoms are physically real or merely a useful idea: is matter really made from atoms, or does it just act as if it were? Atoms are much too small to see, so it was something like arguing about whether a whitewashed house in the distance is made from bricks or concrete. Albert Einstein ended this debate in 1905 when he explained a phenomenon called Brownian motion. When a small particle such as a grain of pollen is suspended in a liquid and observed under a microscope, it is seen to move around in a random, erratic path. Einstein explained that the particle moves because of collisions with atoms. Figments of the imagination can’t produce motion, so Einstein argued that atoms must be real. Today, using devices called scanning tunneling microscopes, we can actually take “photographs” of individual atoms, so this old question has been firmly laid to rest.
Now scientists often argue about whether tiny particles inside the atoms are really made from even smaller particles, called “quarks,” or of even smaller objects called “strings,” or just act as if they were—a debate that mirrors the old argument about atoms.
ANATOMY OF THE ATOM
The atom’s structure closely parallels that of the solar system. A massive central nucleus, analogous to the sun, is orbited by smaller electrons, something like a swarm of planets. The nucleus has a positive electrical charge, the electrons a negative charge, and the electrical attraction between the two holds the whole system together.
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