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Page 1 of 5 KEY CONCEPT Electromagnetic waves have unique traits. BEFORE, you learned NOW, you will learn • Waves transfer energy • Mechanical waves need a medium to travel • How electromagnetic waves differ from mechanical waves • Where electromagnetic waves come from • How electromagnetic waves transfer energy VOCABULARY EXPLORE Electromagnetic Waves electromagnetic wave p. 73 radiation p. 75 How does the signal from a remote control travel? MATERIALS PROCEDURE 1 Turn the TV on and off using the remote control. 2 Work with a partner to try to turn on the TV by aiming the remote control at the mirror. • TV with remote control unit • mirror with stand WHAT DO YOU THINK? How did you have to position the remote control and the mirror in order to operate the TV? Why do you think this worked? An electromagnetic wave is a disturbance in a field. Did you know that you are surrounded by thousands of waves at this very moment? Waves fill every cubic centimeter of the space around you. They collide with or pass through your body all the time. VOCABULARY Create a frame game diagram for the term electromagnetic wave. Most of these waves are invisible, but you can perceive many of them. Light is made up of these waves, and heat can result from them. Whenever you use your eyes to see, or feel the warmth of the Sun on your skin, you are detecting their presence. These waves also allow radios, TVs, and cell phones to send or receive information over long distances. These waves have the properties shared by all waves, yet they are different from mechanical waves in important ways. This second type of wave is called an electromagnetic wave. An electromagnetic wave (ih-LEHKtroh-mag-NEHT-ihk) is a disturbance that transfers energy through a field. Electromagnetic waves are also called EM (EE-EHM) waves. Chapter 3: Electromagnetic Waves 73 D C Page 2 of 5 A field is an area around an object where the object can apply a force—a push or a pull—to another object without touching it. You have seen force applied through a field if you have ever seen a magnet holding a card on the door of a refrigerator. The magnet exerts a pull on the door, even though it does not touch the door. The magnet exerts a force through the magnetic field that surrounds the magnet. When a disturbance occurs in a field rather than in a medium, the wave that results is an electromagnetic wave. How EM Waves Form EM waves come from atomic VISUALIZATION CLASSZONE.COM particles that are electrically Learn more about the charged. Because of their charges, nature of EM waves. these particles can exert a force—a push or a pull—on one another through an electric field. These particles also create the magnetic fields that make magnets work. electric field magnetic field direction of wave When electrically charged particles move quickly, they can start a disturbance or vibration in their electric and magnetic fields. The fields vibrate at right angles to each other, as shown in the diagram above. The EM wave travels in the form of these vibrating fields. As you read in Chapter 1, all waves have the properties of amplitude, wavelength, and frequency. In an EM wave, as the diagram shows, both the electric and the magnetic fields have these three properties. check your reading What are the two types of fields that make up an EM wave? Sources of EM Waves Many of the EM waves present in Earth’s environment come from the Sun. The Sun’s high energy allows it to give off countless EM waves. Other stars give off as many EM waves as the Sun, but because these bodies are so far away, fewer of their EM waves reach Earth. In addition to the Sun, technology is a source of EM waves that humans use for a wide variety of purposes. When EM waves enter a material, the material often responds by giving off more EM waves. Many EM waves in the environment are given off by the surface of the Earth in response to EM waves from the Sun. check your reading D C 74 Unit: Waves, Sound, and Light What is the source of most EM waves in Earth’s environment? Page 3 of 5 Electromagnetic waves can travel in a vacuum. The transfer of energy in the form of EM waves is called radiation (RAY-dee-AY-shuhn). Radiation is different from the transfer of energy through a medium by a mechanical wave. A mechanical wave must vibrate the medium as it moves, and this uses some of the wave’s energy. Eventually, every mechanical wave will give up all of its energy to the medium and disappear. An EM wave can travel without any medium at all—that is, in a vacuum or space empty of matter—and does not lose energy as it moves. In theory, an EM wave can travel forever. reading tip EM waves are also called rays. The words radiation and radiate come from the Latin word radius, which means “ray“ or “spoke of a wheel.“ How EM Waves Travel in a Vacuum Because they do not need a medium, EM waves can pass through outer space, which is a near vacuum. Also, because they do not give up energy in traveling, EM waves can cross the great distances that separate stars and planets. For example, rays from the Sun travel about 150 million kilometers (93 million mi) to reach Earth. Rays from the most distant galaxies travel for billions of years before reaching Earth. In a vacuum, EM waves spread outward in all directions from the source of the disturbance. The waves then travel in a straight line until something interferes with them. The farther the waves move from their source, the more they spread out. As they spread out, there are fewer waves in a given area and less energy is transferred. Only a very small part of the energy radiated from the Sun is transferred to Earth. But that energy is still a great amount—enough to sustain life on the planet. The Speed of EM Waves in a Vacuum In a vacuum, EM waves travel at a constant speed, and they travel very fast—about 300,000 kilometers (186,000 mi) per second. In 1 second, an EM wave can travel a distance greater than 7 times the distance around Earth. Even at this speed, rays from the Sun take about 8 minutes to reach Earth. This constant speed is called the speed of light. The vast distances of space are often measured in units of time traveled at this speed. For example, the Sun is about 8 light-minutes away from Earth. The galaxy shown in the photograph is 60 million light-years from Earth. check your reading The light and other EM waves from this galaxy took approximately 60 million years to reach Earth. How are EM waves used to measure distances in space? Chapter 3: Electromagnetic Waves 75 D C Page 4 of 5 Electromagnetic waves can interact with a material medium. When EM waves encounter a material medium, they can interact with it in much the same way that mechanical waves do. They can transfer energy to the medium itself. Also, EM waves can respond to a change of medium by reflecting, refracting, or diffracting, just as mechanical waves do. When an EM wave responds in one of these ways, its direction changes. When the direction of the wave changes, the direction in which the energy is transferred also changes. reminder Potential energy comes from position or form; kinetic energy comes from motion. Transferring Energy A mechanical wave transfers energy in two ways. As it travels, the wave moves potential energy from one place to another. It also converts potential energy into kinetic energy by moving the medium back and forth. In a vacuum, EM waves transfer energy only by moving potential energy from one place to another. But when EM waves encounter matter, their energy can be converted into many different forms. check your reading In what form do EM waves transfer energy in a vacuum? Wave Behavior SKILL FOCUS How do EM waves interact with matter? Designing experiments PROCEDURE 1 Observe the radiometer on a table or desk. MATERIALS 2 Write a hypothesis in the form of an “If . . . , radiometer then . . . , because . . .” statement to answer the question: What makes the radiometer vanes move? 3 Develop an experiment to test your hypothesis. TIME 30 minutes vanes WHAT DO YOU THINK? • How does light affect the vanes? dark side • Based on your observation of the vanes, does light affect the white and black surfaces differently? If so, how? • How would you modify your design now that you have seen the results? CHALLENGE Based on your observations, what does a radiometer measure? Explain your answer. D C 76 Unit: Waves, Sound, and Light radiometer light side Page 5 of 5 Converting Energy from One Form to Another How EM waves interact with a medium depends on the type of the wave and the nature of the material. For example, a microwave oven uses a type of EM wave called microwaves. Microwaves pass through air with very little interaction. However, they reflect off the oven’s fan and sides. But when microwaves 1 encounter water, such as that inside a potato, their energy is converted into thermal energy. As a result, the potato gets cooked, but the oven remains cool. 1 A device on the oven produces microwaves and sends them toward the reflecting fan. reflecting fan microwave source 2 4 microwaves 3 2 Microwaves are reflected in many directions by the blades of the fan and then again by the sides of the oven. 3 Microwaves move through the air without transferring energy to the air. 4 Microwaves transfer energy to the water molecules inside the potato in the form of heat, cooking the potato. EM waves usually become noticeable and useful when they transfer energy to a medium. You do not observe the microwaves in a microwave oven. All you observe is the potato cooking. In the rest of this chapter, you will learn about different types of EM waves, including microwaves, and about how people use them. check your reading How does microwave cooking depend on reflection? KEY CONCEPTS CRITICAL THINKING 1. How are EM waves different from mechanical waves? 4. Predict What would happen to an EM wave that never came into contact with matter? 2. What are two sources of EM waves in Earth’s environment? 3. How can EM waves transfer energy differently in a material medium as compared to a vacuum? 5. Infer What might be one cause of uneven heating in a microwave oven? CHALLENGE 6. Synthesize EM waves can interact with a medium. How might this fact be used to make a device for detecting a particular type of EM radiation? Chapter 3: Electromagnetic Waves 77 D C