If you stand in front of a campfire, even though you do not actually touch the flames, something emitted from the blazing wood makes itself felt on your face or hands. If you sit in the sun on a summer day, you feel the same sensation of warmth. You know that if you stay out in the sun too long, you will be sunburned.
If you listen to the radio or watch television, the receiver picks up signals sent out from a transmitter and converts them into the program that you hear or see. If you talk on a cell phone, your conversation is likewise converted into signals sent through the air.
These are all examples of electromagnetic radiation, a process whereby energy is carried across space. The various forms of electromagnetic radiation are emitted as the result of periodic variations—variations occurring at regular intervals—in electromagnetic fields. The rate of these variations determines whether the emission is a radio wave, visible light, gamma rays, or some other form of radiation. The complete range of these forms of radiation is known as the electromagnetic spectrum.
All forms of electromagnetic waves have certain characteristics in common. Unlike mechanical waves, such as sound waves and water waves, electromagnetic waves can travel through empty space. All forms travel at the same speed—186,000 miles (300,000 kilometers) per second, the speed of light—when passing through a vacuum. Ordinarily, the energy travels out from its source by spreading equally in all directions, like the radii of a circle.
Like mechanical waves, electromagnetic radiation can be measured in terms of wavelength and frequency. Wavelength is the distance covered in one wave cycle—that is, the distance from wave crest (the point of maximum intensity) to wave crest, or between any two corresponding points. Frequency is the rate at which wave cycles pass through a given point in space—so many cycles per second, for instance. Because the speed of light and other forms of electromagnetic radiation are constant, frequency is directly related to wavelength. The longer the wavelength, the lower the frequency, and vice versa.
Forms of electromagnetic radiation differ from each other mainly in terms of their frequency and wavelength. Each form occupies a range of specific frequencies within the electromagnetic spectrum.
The most familiar kind of radiation is visible light, which makes it possible for us to see the world about us. Long ago, people were puzzled as to just what light was. Some scientists believed in the corpuscular theory, which held that luminous bodies give off tiny particles, or corpuscles, that bounce off objects and are detected when they hit the eye. Others thought light was like the waves formed in a pond when a stone is dropped in it, which travel outward on the surface of the water. If these waves hit a solid obstruction, they are reflected, producing a new series of waves.
One of the basic discoveries about light came in the year 1666 when the celebrated English scientist Sir Isaac Newton held a glass prism over a small hole in a window shutter, through which a beam of sunlight entered the room. The result, instead of a spot of white light, was a band of colors—a spectrum—red at one end, violet at the other. Thus, he showed that what we ordinarily call white light is actually a mixture of colors. Six are now accepted as the principal colors—red, orange, yellow, green, blue, and violet.
Although Newton never expressed a firm opinion as to the nature of light, he leaned toward the corpuscular theory. His great reputation did much to maintain it. Then, in 1690, the Dutch physicist Christiaan Huygens published a wave theory that later became widely accepted.
According to the theory as it stands today, wavelength is what determines the color of light. The longest light waves—more than 40,000 per inch (about 16,000 per centimeter)—produce red light. The shortest, about 71,000 per inch (28,000 per centimeter), produce violet. In between are the wavelengths of the other four colors of the visible spectrum.
At the beginning of the 19th century, scientists found that the spectrum of visible light represents only a narrow range of radiation. Beyond both ends, there are multitudes of other waves, all of which are invisible to the human eye.
Infrared waves have a lower frequency than the red rays of visible light, and are located just below visible light in the electromagnetic spectrum. ("Infra" means "below.") When you feel the warmth of a campfire, you are feeling infrared waves.
The German-born British astronomer Sir William Herschel, famous as the discoverer of the planet Uranus, discovered infrared radiation in 1800. Like Newton, he used a prism to produce a spectrum. Then he placed several thermometers at the various colors, to see which would produce the greatest heating effect. Of all the visible parts, the red light made the temperature go up the most. Then Herschel placed one of the thermometers beyond the red end of the spectrum. The mercury went up even higher, clearly showing that invisible heat rays were present. Herschel performed a whole series of experiments with these "heat rays," and discovered that he could reflect them with mirrors and focus them with lenses, just as visible light is reflected and focused. He also found that these rays were emitted from a bed of hot coals in a fireplace, as well as from the Sun.
While some photographic materials are sensitive to infrared, they do not record rays far beyond the visible spectrum. Other methods must be used, therefore, in studying these rays. Some of the most fundamental work on the infrared region was done by the American astrophysicist Samuel Pierpont Langley. To study infrared rays, Langley invented an electrical device known as the bolometer in 1880. With the bolometer, which detects and measures small quantities of radiant heat—up to one hundred-thousandth of a degree—he studied the infrared spectrum of the Sun well beyond the wavelengths of visible light.
Later, with the bolometer and other, even-more-efficient detectors, scientists found that still longer infrared waves are given off from sources that are not commonly considered hot. Even a glass of ice water emits infrared rays, though rather feebly, in a range of wavelengths up to about 0.0004 inch (0.001 centimeter). If the water is boiling, the emission is stronger and the wavelengths are slightly shorter. For still hotter objects, the total amount of radiation given off increases, and the maximum wavelength becomes shorter and shorter.
This is in accordance with the so-called displacement law discovered by the Nobel Prize–winning German physicist Wilhelm Wien in 1893. According to this law, the product of the wavelength at which a radiating body gives off the greatest energy and the absolute temperature of the body is a constant. As the temperature is increased, the wavelength becomes shorter; and as the temperature drops, the wavelength increases. The principle holds true for visible light as well, allowing astronomers to calculate the temperatures of distant stars and other bodies in space by measuring the wavelengths of their emissions.
Ultraviolet waves are located just beyond the shortest wavelength of the visible spectrum. These waves do not cause heating, but they have chemical effects. Ultraviolet radiation from the Sun is responsible for sunburn, the fading of paints and fabrics, and other phenomena.
Ultraviolet waves were discovered in 1801 by a German physicist, Johann Wilhelm Ritter. Ritter knew that the white compound called silver chloride turns black when exposed to light. He put some of this substance in line with a solar spectrum, out beyond the violet part, in a place where no visible light was shining. The silver chloride blackened even more rapidly than it did in the region where the various colors could be seen. Some unseen radiation was producing the change.
These rays could also be focused and reflected, but they did not penetrate glass very well. By 1852, the English physicist Sir George Stokes had found that the ultraviolet rays could pass through quartz quite easily. He made a device out of quartz lenses and prisms for studying the rays. With the light from an electric spark, he could produce an ultraviolet spectrum ranging to a wavelength about half that which produces the deepest violet light. To detect these invisible rays, Stokes made use of fluorescence. This luminous effect occurs when ultraviolet rays fall on certain chemicals, causing them to shine visibly.
Since silver chloride and similar substances darken when exposed to either visible or invisible light, plates coated with these substances can be used to photograph the invisible spectrum. With refinements in this method, scientists have been able to study the spectrum far into the ultraviolet region.
Radio waves cover a wide range of wavelengths at the low end of the spectrum. Some of the longest radio waves, many miles long, are emitted from electrical power lines. At the other end of the radio-wave spectrum are microwaves, which overlap the region of the longest infrared waves.
The first clue that there might be waves longer than those of the infrared came from the Scottish physicist James Clerk Maxwell. In 1864, he published a series of papers dealing with the nature of light and showed, theoretically, that it is a movement of electrical and magnetic waves. He concluded that these waves could extend far beyond the infrared. One of his suggestions was that a vibrating electrical charge could set up such waves, and that they would travel at the speed of light.
Perhaps the first person to produce electromagnetic waves experimentally was British-born American inventor Elihu Thomson, a science teacher at Philadelphia's Central High School. In 1871, Thomson was experimenting with high-voltage electrical sparks that could jump across a gap of several inches. He took one of the terminals through which the electrical discharge passed, and connected it to a water pipe. He connected the other terminal to a metal tabletop. While the sparks were jumping, Thomson found that he could go to distant parts of the building, hold a knife blade near a metal object, and draw sparks from it. After some other related experiments, Thomson realized that he had confirmed Maxwell's prediction. The energy that produced the sparks at the knifepoint was transmitted from the original spark by waves that traveled through space.
As sometimes happens in scientific history, Thomson's experiments were almost entirely ignored. In 1887, Heinrich Hertz in Germany obtained similar effects with what was essentially the same equipment. The world acclaimed the scientist, and the radiation that was produced came to be known as Hertzian waves. They are now generally called radio waves.
X-RAYS AND GAMMA RAYS
At the other end of the spectrum are X-rays and gamma rays, with wavelengths even shorter than those of ultraviolet waves. X-rays were once called Roentgen rays for their discoverer, Wilhelm Konrad Roentgen, a German professor. In 1895, Roentgen detected mysterious radiation from a Crookes tube, an evacuated glass bulb in which an electrical discharge is produced. This radiation caused some materials to glow by fluorescence. It was possible to record its effect on a photographic plate. But the most striking characteristic of the new rays was that they could pass through many solid materials—wood, paper, and even human flesh. In fact, with these rays, it was possible to see and photograph the bones in the human body.
Shorter waves of radiation are more penetrating than longer waves. The wavelengths of X-rays depend on the voltage of the electricity used to create them: the higher the voltage, the shorter the length. For the X-rays commonly used in medicine and industry, most of the radiation has a wavelength of less than one-billionth of an inch. With some of the large particle accelerators that are used by physicists, X-rays of hundreds of millions of volts of energy are produced, and their wavelengths are still shorter.
Gamma radiation, which is found throughout the universe, generally has even shorter wavelengths and greater penetrating power than X-rays. Half a trillion gamma rays would have to be lined up to equal 1 inch (2.5 centimeters). Thus, the known electromagnetic spectrum extends from the tiny gamma rays all the way down to alternating-current power waves, which are many trillion times as long.
How do light and indeed all forms of electromagnetic radiation arise? This question was not answered until the 20th century, when scientists began to study the workings of atoms.
Light and most of the forms of electromagnetic radiation that people regularly experience originate with changes in the structure of atoms. An atom consists of a positively charged nucleus, around which move negatively charged electrons—from one, in the case of hydrogen, to more than 100, in the case of the heaviest elements known.
Normally, these electrons move in certain regions, or orbits. But when an electron absorbs energy, it may become "excited" and move to a new orbit, farther away from the nucleus. Scientifically, it is said to have shifted to a higher energy level, a phenomenon known as electron jump. The more energy the atom absorbs, the greater the electron jump. After an electron has been thus shifted, it tends to fall back to its former position. When it does, the electron releases the extra energy it absorbed.
When an atom absorbs outside energy, electrons may be rapidly forced out of their orbits and fall back, over and over again. As they oscillate in this way, the atom releases energy very rapidly in the form of radiation. The intensity of the radiation depends exclusively on the amount of energy that an atom has absorbed—it can radiate only that specific amount. The frequency of the radiation depends on the frequency of the oscillation.
Radiation may also arise from the fusion of atoms or the decay of radioactive elements. Gamma rays are products of natural radioactivity and have been produced in the laboratory. (Most forms of radioactive radiation are created when particles are emitted from nuclear reactions; these radiations are not considered part of the electromagnetic spectrum.)
The wave theory originally proposed by Huygens won wide acceptance in the 19th century. Scientists were sure that light and other forms of electromagnetic radiation consisted of waves rippling through space. They drew sharp distinctions between the wave character of radiation and the particle character of matter.
Then, about 1900, the work of Max Planck in Germany, and later that of Niels Bohr in Denmark, showed that light and other forms of electromagnetic radiation sometimes behave in ways that the wave theory cannot explain. In certain respects, light acts as if it were a stream of tiny packets of energy called photons. A photon is the smallest amount of light energy possible. Many other forms of radiation—especially those that, like light, are emitted from excited atoms—seem to show the same property: radiant energy is emitted and absorbed in separate, particle-like units, or quanta.
These ideas of Planck and Bohr were eventually united in a revolutionary theory of atomic structure, although the overwhelming weight of research and experimentation still seemed to uphold the wave nature of light. By itself, neither the wave theory nor the quantum theory could satisfactorily account for all the various properties and behaviors of light and other forms of radiation. A compromise was introduced about 1925, when a French physicist, Louis Victor de Broglie, and an Austrian theoretical physicist, Erwin Schrödinger, originated the theory of wave mechanics.
According to the theory of wave mechanics, all forms of matter and energy have both wavelike and particle-like qualities, but the two aspects never appear together under the same conditions. In transit—as it moves from place to place—electromagnetic radiation behaves like a system of waves; in empty space, it has a fixed speed. Its wavelengths can be measured by a variety of methods. In many ways, its behavior is entirely consistent with the theory that electromagnetic radiation is a wave disturbance. But in their emission from electrons in an atom and in their absorption by—and other reactions with—atoms in their path, light and other forms of electromagnetic radiation behave more like streams of very small particles of energy.
Even stranger, perhaps, electrons and other very small particles sometimes behave like waves. Beams of electrons and many other elementary particles have been diffracted to produce interference patterns just like those of a conventional wave. From these patterns, the wavelength of the electron has been measured and found to agree with the predictions of wave-mechanics theory. Therefore, the seemingly paradoxical ideas of Schrödinger and de Broglie have been generally accepted by science.
The theory of wave mechanics is complex, and it would take a great deal of space to explain it in full. It may be enough to remember that, instead of saying that light or any other kind of radiation consists of waves or particles, physicists generally recognize that these radiations behave like waves in certain respects and like particles in others, a phenomenon known as wave-particle duality.
USES OF ELECTROMAGNETIC RADIATION
The uses of visible light are obvious enough. One can readily imagine what the world would be like without light. But it would be difficult to catalog all the uses that people have invented for other forms of electromagnetic radiation.
Not long after the discovery of radio waves, for example, several people realized that these waves might make it possible to send messages between distant points, without any wires to carry them. The first person to succeed in doing this was a young Italian engineer, Guglielmo Marconi. After some preliminary experiments in which he sent signals across a vegetable patch near his home, Marconi improved his equipment and on December 12, 1901, sent the letter S in the Morse telegraphic code across the Atlantic Ocean from England to Newfoundland. From this humble beginning came such developments as television, radio broadcasting, and radar.
Today, most of the electronic communication systems and devices that people use every day—including broadcast radio and television, two-way radios, cell phones, pagers, remote controls, and wireless computer networks—rely on radio waves. The waves used in broadcasting are about 1,000 feet (300 meters) in length. Still longer waves, more than 0.62 mile (1 kilometer) in length, are sometimes used in transmitting radio messages across the ocean. In microwave ovens, radio waves of microwave length cook food by causing molecules of liquid to vibrate, producing heat.
People have countless uses for infrared radiation. Household appliances such as toasters and electric broilers cook food with infrared waves. Other heat-producing appliances, such as heat lamps and electric space heaters, use infrared wavelengths as well. Cameras and other devices that detect infrared radiation are used to "see" objects in the dark and to monitor mechanical systems and buildings for heat loss. Thermography (infrared heat imaging) is also used in medicine to locate areas of increased heat that could indicate soft-tissue injuries and blood-flow problems. Infrared images taken from satellites are used in climate and weather research, to detect changes in the surface temperature of ocean and land areas.
Fluorescent lighting is a highly efficient light source and the most important application of ultraviolet light. Although ultraviolet rays are invisible, they cause substances known as phosphors to emit visible light, or fluoresce. A fluorescent lamp is basically an ultraviolet lamp made of glass that prevents the ultraviolet rays from escaping. The inside of the bulb is coated with a thin layer of fluorescent material that emits visible light when struck by the ultraviolet rays. Certain wavelengths of ultraviolet light are able to kill or inactivate bacteria. For this reason, ultraviolet lamps are used in hospital operating rooms and in some manufacturing processes requiring sterile air.
At the short end of the electromagnetic spectrum, physicians use X-rays to reveal inner parts of the body and diagnose disease. X-rays are also used to detect tiny flaws in materials and even to analyze the structure of matter. A beam of X-rays, passed through a crystal in which the atoms are arranged in regular layers, is scattered and forms a pattern of spots characteristic of that particular kind of crystal on a photographic film. This is called X-ray diffraction. Because high-voltage X-rays can kill cells, they are sometimes used to destroy cancer cells.
Gamma rays are used in some methods of food irradiation. In this process, food is briefly exposed to a burst of radiation to kill bacteria and parasites that could cause food-borne diseases. Many foods—including some meats, seafood, fruits, vegetables, and spices—are irradiated. The food is not radioactive, and its nutritional value is basically not changed.