How do waves interact with objects? How do waves behave when they move between two media? How do waves interact with other waves?

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1 CHAPTER 20 3 Wave Interactions SECTION The Energy of Waves BEFORE YOU READ After you read this section, you should be able to answer these questions: How do waves interact with objects? How do waves behave when they move between two media? How do waves interact with other waves? National Science Education Standards PS 3a Why Do Waves Reflect? A reflection occurs when a wave bounces back after hitting a barrier. All waves can be reflected. Light waves reflecting off an object allow you to see that object. For example, light waves from the sun reflecting off the moon allow you to see the moon. Sound wave can also reflect. Sound waves reflecting off a barrier are called an echo. STUDY TIP In your science notebook, define each new vocabulary word. Include sketches illustrating reflection, refraction, diffraction, and both kinds of interference. The waves in this photograph were formed by drops of water that fell into a container of water. When the waves caused by the drops of water hit one side of the container, they reflect off. The shape of the reflected waves is opposite that of the waves that struck the side of the tank. Waves are not always reflected when they hit a barrier. Sometimes they pass through a substance. When a wave passes through a substance, it is transmitted. Light waves transmitted through a glass window allow light to enter a room. Light waves transmitted through eyeglasses allow the wearer to see through them. Critical Thinking 1. Infer How does your reflection in a bathroom mirror look when you raise you right arm? Interactive Textbook 375 The Energy of Waves

2 SECTION 3 Wave Interactions continued 2. Describe What is diffraction? Why Do Waves Diffract? Diffraction is the bending of waves around a barrier or through an opening. Waves usually travel in a straight line. When a wave reaches the edge of an object or an opening in a barrier, it may curve or bend. The amount of diffraction of a wave depends on its wavelength and the size of the barrier opening. Sound waves are relatively long. You can hear voices from one classroom diffract through the opening of a door into another classroom. Light waves are relatively short. You cannot see who is speaking in the other classroom. 3. Describe Suppose the opening in the lower figure were made larger. What would happen to the shape of the diffracted wave? If the barrier or opening is larger than the wavelength of the wave, there is only a small amount of diffraction. If the barrier or opening is the same size or smaller than the wavelength of an approaching wave, the amount of diffraction is large. Why Do Waves Refract? Refraction is the bending of a wave as the wave passes from one medium to another. The wave changes speed as it passes from one material to the other. The change in speed causes the wavelength to change. The resulting wave bends and travels in a new direction. 4. Describe What happens to a wave because of refraction? This light wave is refracted as it passes into a new medium. The light wave is passing from air into water. The wave is refracted because the speed of the wave changes. Interactive Textbook 376 The Energy of Waves

3 SECTION 3 Wave Interactions continued REFRACTION OF DIFFERENT COLORS When light waves from the sun pass through a droplet of water in the air, the light is refracted. The different colors of light travel at different speeds through the drop. Therefore, the different colors are refracted by different amounts. The light is dispersed, or spread out, into its separate colors. The result is a rainbow. White light (from the sun) Red Yellow 5. Describe What does light do when it disperses? Green Violet Glass prism White light is separated into its component colors when it passes through a prism. The red light is refracted the least. The violet light is refracted the most. What Is Wave Interference? All matter has volume. Therefore, objects cannot be in the same space at the same time. However, waves are made up of energy, not matter. So, more than one wave may be in the same space at the same time. Two waves can meet, share the same space, and pass through each other. When two or more waves meet and share the same space, they overlap. Interference is the combination of two or more waves to form a single wave. CONSTRUCTIVE INTERFERENCE Constructive interference occurs when the crests of one wave overlap with the crests of another wave or waves. The troughs of both waves will also overlap. The energy of the waves adds together to make a higher-energy wave. The new wave has higher crests, deeper troughs, and, therefore, higher amplitude. 6. Identify The order of the dispersed colors can be remembered by the mnemonic ROY G. BIV. Draw a line coming from the prism that shows the direction that orange would move in. 7. Identify What is the combination of two or more waves to form a single wave called? 8. Describe What does the medium do after the waves have overlapped and are continuing their movement? Waves approaching Waves overlapping Waves continuing Constructive Interference When waves combine by constructive interference, the combined wave has a larger amplitude. Interactive Textbook 377 The Energy of Waves

4 SECTION 3 Wave Interactions continued DESTRUCTIVE INTERFERENCE Destructive interference occurs when the crests of one wave overlap with the troughs of another wave. The energy of the new wave is less than the energy of both waves. The new wave has lower amplitude than the original waves. If a crest and trough of the same amplitude meet and cancel, the result is no wave at all. 9. Describe What parts of a wave overlap during destructive interference? Waves approaching Waves overlapping Waves continuing Destructive Interference When two waves with the same amplitude combine by destructive interference, they cancel each other out. How Is a Standing Wave Created? In a standing wave, the pattern of vibrations makes it appear as if the wave is standing still. A standing wave is caused by interference between a wave and a reflected wave. For example, pluck a guitar string. The string makes a standing wave like the top wave shown in the figure below. 10. Describe What causes a standing wave? 11. Identify Draw two arrows in the bottom figure that show the locations of destructive interference. Destructive interference Constructive interference A rope vibrating at certain frequencies can create a standing wave. The initial wave travels down the rope and will be reflected back when it reaches the end. In a standing wave, certain parts of the wave are always at the rest position. This point is caused by destructive interference between the waves. Constructive interference can be seen at points in the wave where there is large amplitude. Remember, a standing wave only looks as if it is standing still. Waves are actually moving in two directions. Standing waves can be formed with transverse waves or with longitudinal waves. Interactive Textbook 378 The Energy of Waves

5 SECTION 3 Wave Interactions continued RESONANCE Resonant frequencies are the frequencies at which standing waves are created. Resonance occurs when two objects naturally vibrate at the same frequency. The resonating object absorbs energy from the vibrating object and vibrates also. For example, when a guitar string is plucked, the wood body vibrates at the same frequency as the string. When you pluck the guitar string, you hear a musical note. The vibrating wood body makes sound waves in the air. The sound waves that reach your ear make parts of your ear vibrate, so you hear the sound of the note. 12. Describe When does resonance occur? Summary of Wave Interactions Constructive Interference Destructive Interference Standing Wave crests of two waves overlap crest and trough of two waves overlap crests and troughs overlap such that the wave looks to be standing still Refraction Interference Interaction between two mediums Interaction with another wave Wave Interactions bounces back Reflection Interaction with a barrier bends around Diffraction 13. Identify What two interactions can occur when a wave strikes a barrier? Interactive Textbook 379 The Energy of Waves

6 Section 3 NSES PS 3a SECTION VOCABULARY diffraction a change in the direction of a wave when the wave finds an obstacle or an edge, such as an opening interference the combination of two or more waves that results in a single wave reflection the bouncing back of a ray of light, sound, or heat when the ray hits a surface that it does not go through refraction the bending of a wavefront as the wavefront passes between two substances in which the speed of the wave differs resonance a phenomenon that occurs when two objects naturally vibrate at the same frequency; the sound produced by one object causes the other object to vibrate. standing wave a pattern of vibration that simulates a wave that is standing still 1. Describe Suppose two students grab an end of a rope. Both shake the rope once in an upward direction to create crests traveling at each other. What would you see when the crests meet? What type of interference does this show? 2. Describe Suppose two students grab an end of a rope. Both shake the rope once, but one makes a crest and the other a trough traveling at each other. What would you see when the crest and trough meet? What type of interference does this show? 3. Identify Using the vocabulary words above, fill in the table below by writing which wave interaction most likely caused the situation described. Wave situation Wave interaction The image of an object in a mirror A straight pencil appears to bend when the bottom half is placed in a glass of water Two rocks dropped in the water a meter apart produce a large wave centered between them A radio turned on in one classroom can be heard down the hall in a second classroom Interactive Textbook 380 The Energy of Waves

7 Physical Science Answer Key continued 4. Mechanical waves need a medium to transfer their energy, while electromagnetic waves don t. 5. transverse and longitudinal waves 6. at right angles to the direction of motion 7. The wave vibrations are at right angles to the direction the wave is traveling. 8. back and forth along the path that the wave moves 9. Shake the spring up and down. 10. Densely packed group of particles should be circled. 11. water and sand in the shallow water 12. mechanical wave 1. Energy moves in the direction of the wave away from the wave source. Energy is transferred from one particle to the particle next to it. 2. Wave source Wave energy transfer (electromagnetic wave or mechanical wave) Light emitted from a light bulb Sound coming from a violin Rock dropped in a pond electromagnetic wave mechanical wave mechanical wave Wave type (transverse wave, longitudinal wave, or surface wave) transverse wave longitudinal wave transverse wave 3. The ribbon is in the same position on the first loop of the spring. 4. Compression: Particles are crowded toward each other. Crest: Particles are at their highest point. Trough: Particles are at their lowest point. Rarefaction: Particles are spread away from each other. SECTION 2 PROPERTIES OF WAVES 1. the maximum distance the particles of a wave vibrate away from their rest position 2. It took more energy to make the wave. 3. one-half wavelength 4. the group of waves with short wavelengths hertz, or 30 Hz 6. 0:20 7. those with the highest frequency 8. v f 30 m 20/s 600 m/s 1. The amplitude is 0.5 meters. 2. Wave description Wave energy High amplitude high Low frequency low Low wavelength high 3. One way is to compress coils closer together (make short-wavelength waves) before releasing the spring. The other is to make the frequency high. 4. v f 100 m 25/s 2,500 m/s 5. v 5,000 m/s 40 m f 125/s SECTION 3 WAVE INTERACTIONS 1. It looks like the opposite. When you raise your right arm, it looks like you raised your left arm. 2. the bending of waves around a barrier or through an opening 3. It would become straighter (have less diffraction). 4. It slows down and bends to travel in a new direction. 5. spreads out 6. A line should be drawn between red and yellow. 7. interference 8. The medium returns to its rest, or undisturbed, position. 9. crests and troughs 10. interference between a wave and a reflected wave 11. Arrows should be drawn at the two cancellation points, not at the end points. 12. when two objects naturally vibrate at the same frequency 13. reflection and diffraction 1. When the crests meet, a wave with larger amplitude is formed. This is constructive interference. 2. When the crest and trough meet, a wave with smaller amplitude is formed. This is destructive interference. Interactive Textbook Answer Key 93 Physical Science

8 Physical Science Answer Key continued 3. Wave situation The image of an object is seen in a mirror. A straight pencil appears to bend when the bottom half is placed in a glass of water. Two rocks dropped in the water a meter apart produce a large wave centered between them. A radio turned on in one classroom can be heard down the hall in a second classroom. Wave interaction reflection refraction interference diffraction 3. They can damage the hair cells in the cochlea. Damaged hair cells do not grow back, so damage can result in permanent hearing loss. 4. The compressions are regions where particles are packed more closely. The rarefactions are regions where particles are spread farther apart. 5. a medium 6. Small vibrations may not be increased enough to be heard. Chapter 21 The Nature of Sound SECTION 1 WHAT IS SOUND? 1. the complete back-and-forth motion of an object 2. Two areas where particles are clustered close together should be circled. 3. The paths are the same. 4. any substance that a wave can travel through; yes 5. tiny hair cells 6. The eardrum makes the hammer vibrate. This makes the anvil vibrate and the stirrup vibrate. 7. Sound is made by making an object vibrate; detecting a sound means hearing the sound. 8. ringing or buzzing in the ear 9. They can damage the inner ear and cause permanent hearing loss. 10. Stay away from loud sounds; use earplugs when needed; lower the volume when using headphones. 1. The particles in the air vibrate back and forth. They do not move through space but vibrate around a resting position. 2. Organ in ear Work done by the organ Stirrup It vibrates the oval window in the entrance to the inner ear. Cochlea Vibrations of the liquid inside it cause hair cells to bend. Hair cells When they bend, nerves are stimulated to send electrical signals to the brain. Ear canal It funnels sound waves into the middle ear. Eardrum Membrane stretched over the opening to the middle ear vibrates with the sound waves. SECTION 2 PROPERTIES OF SOUND 1. solids and liquids 2. equal to 10 compressions or rarefactions per second 3. ultrasonic sounds 4. The source of the sound is moving relative to the listener. 5. The waves at the rear of the car are farther apart; they have lower frequency than sound waves at the front of the car. 6. When energy... Amplitude... Loudness......increases...increases...increases...decreases...decreases...decreases 7. decibel 8. Crests represent compressions, and troughs represent rarefactions. 9. The graph on the left has lower amplitude than the graph on the right, but the wavelengths are the same. 10. The graph on the left has higher frequency and shorter wavelength than the graph on the right. 1. The compressions and rarefactions of the sound wave are pushed closer together. This makes the frequency higher. Therefore, the pitch is higher. 2. Medium Relative speed A glass window A glass of lemonade Air on a cold winter day faster faster slower 3. The drum struck with a lot of energy will be louder. The more energy added to the vibration, the farther the particles in the medium move from their rest position. The amplitude of the wave is larger, and the sound is louder. Interactive Textbook Answer Key 94 Physical Science