Chapter 8 Electromagnetism and EM Waves (Section 5)

8.5 Electromagnetic Waves Eyes, radios, televisions, radar, x-ray machines, microwave ovens, heat lamps. . . . What do all of these things have in common? They all use electromagnetic waves (EM waves). EM waves occupy prominent places both in our daily lives and in our technology. These waves are also involved in many natural processes and are essential to life itself. In the rest of this chapter, we will discuss the nature and properties of electromagnetic waves and look at some of their important roles in today’s world.

8.5 Electromagnetic Waves As the name implies, EM waves involve both electricity and magnetism. The existence of these waves was first suggested by 19th century physicist James Clerk Maxwell while he was analyzing the interactions between electricity and magnetism.

8.5 Electromagnetic Waves Consider the two principles of electromagnetism stated in Section 8.3: Let’s say that an oscillating electric field is produced at some place. The electric field switches back and forth in direction while its strength varies accordingly. This oscillating electric field will induce an oscillating magnetic field in the space around it.

8.5 Electromagnetic Waves But the oscillating magnetic field will then induce an oscillating electric field. This will then induce an oscillating magnetic field and so on in an endless “loop”: The principles of electromagnetism tell us that a continuous succession of oscillating magnetic and electric fields will be produced.

8.5 Electromagnetic Waves These fields travel as a wave— an EM wave. Electromagnetic Wave: A transverse wave consisting of a combination of oscillating electric and magnetic fields. Electromagnetic waves are transverse waves because the oscillation of both of the fields is perpendicular to the direction the wave travels.

8.5 Electromagnetic Waves The figure shows a “snapshot” of an EM wave traveling to the right. The three axes are perpendicular to each other. In this particular case, the electric field is vertical.

8.5 Electromagnetic Waves As the wave travels by a given point in space, the electric field oscillates up and down, the way a floating petal oscillates on a water wave. The magnetic field at the point oscillates horizontally.

8.5 Electromagnetic Waves Electromagnetic waves do differ from mechanical waves in two important ways. First, they are a combination of two waves in one: an electric field wave and a magnetic field wave. These cannot exist separately. Second, EM waves do not require a medium in which to travel. They can travel through a vacuum: the light from the Sun does this. They can also travel through matter. Light through air and glass, and x-rays through your body are common examples.

8.5 Electromagnetic Waves Electromagnetic waves travel at an extremely high speed. Their speed in a vacuum, called the “speed of light” because it was first measured using light, is represented by the letter c. Its value is c = 299,792,458 m/s (speed of light) or c = 3×108 m/s (approximately) = 300,000,000 m/s = 186,000 miles/s (approximately)

8.5 Electromagnetic Waves All of the parameters introduced for waves in Chapter 6 apply to EM waves. The wavelength can be readily identified in the figure.

8.5 Electromagnetic Waves The amplitude is the maximum value of the electric field strength. The equation v = fl holds with v replaced by c. There is an extremely wide range of wavelengths of EM waves, from the size of a single proton, about 10–15 meters, to almost 4,000 kilometers for one type of radio wave. The corresponding frequencies of these extremes are about 1023 hertz and 76 hertz, respectively. Most EM waves used in practical applications have extremely high frequencies compared to sound.

8.5 Electromagnetic Waves Example 8.2 An FM radio station broadcasts an EM wave with a frequency of 100 megahertz. What is the wavelength of the wave? The prefix mega stands for 1 million. Therefore, the frequency is 100 million hertz.

8.5 Electromagnetic Waves Electromagnetic waves are named and classified according to frequency. In order of increasing frequency, the groups, or “bands,” are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma rays (G-rays). Use of the word radiation instead of waves is not significant here. This is called the electromagnetic spectrum.

8.5 Electromagnetic Waves Notice that the groups overlap. For example, a 1017-hertz EM wave could be ultraviolet radiation or an x-ray. In cases of overlap, the name applied to an EM wave depends on how it is produced.

8.5 Electromagnetic Waves We will briefly discuss the properties of each group of waves in the electromagnetic spectrum— how they are produced, what their uses are, and how they can affect us The great diversity of uses of EM waves arises from the variety of ways in which they can interact with different kinds of matter. All matter around us contains charged particles (electrons and protons), so it seems logical that EM waves can affect and be affected by matter.

8.5 Electromagnetic Waves The oscillating electric field can cause AC currents in conductors; it can stimulate vibration of molecules, atoms, or individual electrons; or it can interact with the nuclei of atoms Which sort of interaction occurs, if any, depends on the frequency (and wavelength) of the EM wave and on the properties of the matter through which it is traveling— its density, molecular and atomic structure, and so on

8.5 Electromagnetic Waves In principle, an electromagnetic wave of any frequency could be produced by forcing one or more charged particles to oscillate at that frequency. The oscillating field of the charges would initiate the EM wave. The “lower-frequency” EM waves (radio waves and microwaves) are produced this way: A transmitter generates an AC signal and sends it to an antenna.

8.5 Electromagnetic Waves At higher frequencies, this process becomes increasingly difficult. Electromagnetic waves above the microwave band are produced by a variety of processes involving molecules, atoms, and nuclei. Note that charged particles are present in all of these processes.

8.5 Electromagnetic Waves There is one other factor to keep in mind: electromagnetic waves are a form of energy. Energy is needed to produce EM waves, and energy is gained by anything that absorbs EM waves. The transfer of heat by way of radiation is one example.

8.5 Electromagnetic Waves Radio Waves Radio waves, the lowest frequency EM waves, extend from less than 100 hertz to about 109 Hz: 1 billion hertz or 1,000 megahertz

8.5 Electromagnetic Waves Radio Waves Within this range are a number of frequency bands that have been given separate names— for example, ELF (extremely low frequency), VHF (very high frequency), and UHF (ultrahigh frequency) Most frequencies are given in kilohertz (kHz) or megahertz (MHz). Sometimes radio waves are classified by wavelength: long wave, medium wave, or short wave

8.5 Electromagnetic Waves Radio Waves As mentioned earlier, radio waves are produced using AC with the appropriate frequency. Radio waves propagate well through the atmosphere, which makes them practical for communication. Lower-frequency radio waves cannot penetrate the upper atmosphere, so higher frequencies are used for space and satellite communication. Only the very lowest frequencies can penetrate ocean water.

8.5 Electromagnetic Waves Radio Waves By far the main application of radio waves is in communication. The process involves broadcasting a certain frequency of radio wave with sound, video, or other information “encoded” in the wave. The radio wave is then picked up by a receiver, which recovers the information.

8.5 Electromagnetic Waves Radio Waves Sometimes, this is a one-way process (commercial AM and FM radio and television), but in most other applications, it is two-way: Each party can broadcast as well as receive. Narrow frequency bands are assigned for specific purposes. For example, frequencies from 88 to 108 megahertz (88 million hertz to 108 million hertz) are reserved for commercial FM radio. There are dozens of bands assigned to government and private communication.

8.5 Electromagnetic Waves Microwaves The next band of EM waves, with frequencies higher than those of radio waves, is the microwave band. The frequencies extend from the upper limit of radio waves to the lower end of the infrared band, about 109 to 1012 hertz. The wavelengths range from about 0.3 m to 0.3 mm.

8.5 Electromagnetic Waves Microwaves One use of microwaves is in communication. Bluetooth and WiFi signals that interconnect computers, cell phones, and other devices are microwaves. Early experiments with microwave communication led to the most important use of microwaves, radar (radio detection and ranging), after the discovery that microwaves are reflected by the metal in ships and aircraft. Radar is echolocation using microwaves.

8.5 Electromagnetic Waves Microwaves The time it takes microwaves to make a round-trip from the transmitter to the reflecting object and back is used to determine the distance to the object. Radar systems are quite sophisticated: Doppler radar can determine the speed of an object moving toward or away from the transmitter by measuring the frequency shift of the reflected wave. Such radars are essential tools for air traffic control and monitoring severe weather.

8.5 Electromagnetic Waves Microwaves Since 2005, the Cassini spacecraft has used imaging radar to penetrate the dense, perpetual smog that envelopes Titan, Saturn’s largest moon, and to map its surface topology. Similar radar equipment placed in orbit around Earth is used to form images of its surface, for such purposes as monitoring changes in the global environment and searching for geological formations (ancient craters, for example) and archaeological sites.

8.5 Electromagnetic Waves Microwaves Microwaves have gained wide acceptance as a way to cook food. The goal of cooking is to heat the food, in other words, increase the energies of the molecules in the food. Conventional ovens heat the air around the food and rely on conduction (in solids) or convection (in liquids) to transfer the heat throughout the food.

8.5 Electromagnetic Waves Microwaves Microwave ovens send microwaves (typically with f = 2,450 megahertz and l = 0.122 meters) into the food. The microwaves penetrate the food and raise the energies of the molecules directly. Recall that water consists of polar molecules—they have a net positive charge on one side and a net negative charge on the other side.

8.5 Electromagnetic Waves Microwaves The electric field of a microwave exerts forces on the two sides of the water molecules in food. These forces are in opposite directions and twist the molecule. Because the electric field is oscillating, the molecules are alternately twisted one way and then the other.

8.5 Electromagnetic Waves Microwaves This process increases the kinetic energy of the molecules and thereby raises the temperature of the food. Cooking with microwaves is fast because energy is given directly to all of the molecules. It does not rely completely on the conduction of heat from the outside to the inside of the food— a much slower process.

8.5 Electromagnetic Waves Infrared Infrared radiation (IR; also called infrared light) occupies the region between microwaves and visible light in the electromagnetic spectrum. The frequencies are from about 1012 hertz to about 4×1014 hertz (400,000,000 megahertz). The wavelengths of IR range from approximately 0.3 to 0.00075 millimeters.

8.5 Electromagnetic Waves Infrared Infrared radiation is ordinarily the main component of heat radiation. Everything around you is both absorbing and emitting infrared radiation, just as you are. The warmth you feel from a fire or heat lamp is the result of your skin absorbing the IR. Infrared radiation is constantly emitted by atoms and molecules because of their thermal vibration. Absorption of IR by a cooler substance increases the vibration of the atoms and molecules, thus raising the temperature.

8.5 Electromagnetic Waves Infrared Infrared radiation is commonly used in wireless remote-control units for televisions and for short-distance wireless data transfer between such devices as personal digital assistants (PDAs) and laptop computers. These units emit coded IR that is detected by other devices. In this capacity, IR is used much like radio waves. Another use of IR is in lasers; some of the most powerful ones in use emit infrared light.

8.5 Electromagnetic Waves Visible Light Visible light is a very narrow band of frequencies of EM waves that happens to be detectable by human beings. Certain specialized cells in the eye, called rods and cones, are sensitive to EM waves in this band. They respond to visible light by transmitting electrical signals to the brain, where a mental image is formed. The visible ranges of some animals such as hummingbirds and bees extend into the ultraviolet band. Some flowers that seem plain to humans are quite attractive to these nectar eaters.

8.5 Electromagnetic Waves Infrared Visible light is a component of the heat radiation emitted by very hot objects. About 44 percent of the Sun’s radiation is visible light: it glows white hot Incandescent light bulbs produce visible light in the same way. Fluorescent and neon lights use excited atoms that emit visible light.

8.5 Electromagnetic Waves Infrared Within the narrow band of visible light, the different frequencies are perceived by people as different colors. The lowest frequencies of visible light, next to the infrared band, are perceived as the color red. The highest frequencies are perceived as violet.

8.5 Electromagnetic Waves Infrared The table shows the approximate frequencies and wavelengths of the six main colors in the rainbow.

8.5 Electromagnetic Waves Infrared Note how narrow the frequency band is: The highest frequency of light we can see is less than twice the lowest. By comparison, the range of frequencies of sound that can be heard is huge: The highest is 1,000 times the lowest.

8.5 Electromagnetic Waves Infrared Most colors that you see are combinations of many different frequencies. White represents the extreme: One way to produce white light is to combine equal amounts of all frequencies (colors) of light.

8.5 Electromagnetic Waves Infrared Rainbow formation involves reversing the process: White light is separated into its component colors. When no visible light reaches the eye, we perceive black. In our daily lives, visible light is the most important of all electromagnetic waves.

8.5 Electromagnetic Waves Ultraviolet Radiation Ultraviolet (UV) radiation, also called ultraviolet light, is a band of EM waves that begins just above the frequency of violet light and extends to the x-ray band. The frequency range is from about 7.5×1014 hertz to 1018 hertz.

8.5 Electromagnetic Waves Ultraviolet Radiation Ultraviolet light is also part of the heat radiation emitted by very hot objects. About 7 percent of the radiation from the Sun is UV. This part of sunlight is responsible for suntans and sunburns. Ultraviolet radiation does not warm the skin as much as IR, but it does trigger a chemical process in the skin that results in tanning. Overexposure leads to sunburn as a short-term effect, and repeated overexposure during a person’s lifetime increases the chance of developing skin cancer.

8.5 Electromagnetic Waves Ultraviolet Radiation Some substances undergo fluorescence when irradiated with UV: They emit visible light. The inner surfaces of fluorescent lights are coated with such a substance. The UV emitted by excited atoms in the tube strikes the fluorescent coating, and visible light is produced. The same process is used in plasma TVs.

8.5 Electromagnetic Waves Ultraviolet Radiation Some fluorescent materials appear to be colorless under normal light and can be used as a kind of invisible ink. They can be seen under a UV lamp but are invisible otherwise.

8.5 Electromagnetic Waves Ultraviolet Radiation UV radiation has many practical applications. For example, it is used as an investigative tool at crime scenes to help identify bodily fluids such as blood and bile. Ultraviolet lights are used by entomologists to attract and collect nocturnal insects for cataloging and study.

8.5 Electromagnetic Waves Ultraviolet Radiation Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. And, increasingly, ultraviolet lasers are finding use in many fields from metallurgy (engraving) to medicine (dermatology and optical keratectomy) to computing (optical data storage).

8.5 Electromagnetic Waves X-Rays The next higher frequency electromagnetic waves are x-rays. They extend from about 1016 to 1020 hertz. An important feature of x-rays is that their range of wavelengths (about 10–8 to 10–11 meters) includes the size of the spacing between atoms in solids.

8.5 Electromagnetic Waves X-Rays X-rays are partially reflected by the regular array of atoms in a crystal and so can be used to determine the arrangement of the atoms. X-rays also travel much greater distances through most types of matter compared to UV, visible light, and other lower-frequency EM waves.

8.5 Electromagnetic Waves X-Rays X-rays are produced by smashing high-speed electrons into a “target” made of copper, tungsten or some other metal. The electrons spontaneously emit x-rays as they are rapidly decelerated on entering the metal. X-rays are also emitted by some of the atoms excited by the high-speed electrons.

8.5 Electromagnetic Waves X-Rays Medical and dental “x-ray” photographs are made by sending x-rays through the body. Typically, x-rays with frequencies between 3.6×1018 hertz and 12×1018 hertz are used. As x-rays pass through the body, the degree to which they are absorbed depends on the material through which they pass.

8.5 Electromagnetic Waves X-Rays Tissue containing elements with relatively large atomic numbers (Z), such as calcium (Z = 20), tend to absorb x-rays more effectively than those that contain predominantly light elements such as carbon (Z = 6), oxygen (Z = 8), or hydrogen (Z = 1). Lead, with atomic number 82, is a particularly good shield for blocking x radiation.

8.5 Electromagnetic Waves X-Rays Bones, which are rich in calcium, absorb x-rays better than soft tissue such as muscle or fat, and hence they show up more clearly on x-rays.

8.5 Electromagnetic Waves X-Rays X-rays (and gamma rays) can be harmful because they are ionizing radiation—radiation that produces ions as it passes through matter. Such radiation can “kick” electrons out of atoms, leaving a trail of freed electrons and positive ions. This process can break chemical bonds between atoms in molecules, thereby altering or destroying the molecule.

8.5 Electromagnetic Waves X-Rays Living cells rely on very large, sophisticated molecules for their normal functioning and reproduction. Disruption of such molecules by ionizing radiation can kill the cell or cause it to mutate, perhaps into a cancer cell. The human body can (and does) routinely replace dead cells, but massive doses of x-rays or other ionizing radiation can overwhelm this process and cause illness, cancer, or death.

8.5 Electromagnetic Waves X-Rays Because medical x-rays are the largest source of artificially produced radiation in the United States, comprising about 10 percent of the total annual radiation dose for the average resident, it is little wonder that protecting the public from unnecessary exposure to damaging radiation in diagnostic radiology is one of the greatest challenges to health and radiological physicists

8.5 Electromagnetic Waves Gamma Rays The highest-frequency EM waves are gamma rays (g-rays). The frequency range is from about 3×1019 hertz to beyond 1023 hertz. The wavelength of higher-frequency gamma rays is about the same distance as the diameter of individual nuclei. Gamma rays are emitted in a number of nuclear processes: radioactive decay, nuclear fission, and nuclear fusion, to name a few

8.5 Electromagnetic Waves Gamma Rays This concludes our brief look at the electromagnetic spectrum. Even though the various types of waves are produced in different ways and have diverse uses, the only real difference in the waves themselves is their frequency and, therefore, their wavelength.