23.3 Wave Motion. Chapter 23. When a wave encounters objects WAVES

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1 23.3 Wave Motion Sometimes your car radio fades out. Why? It s because the radio waves are affected by objects. For example, if you drive into a tunnel, some or all of the radio waves get blocked. In this section, you will learn how waves move and discover what happens when they encounter objects or collide with other waves. When a wave encounters objects Wave fronts The direction a wave moves The four wave interactions A wave front is the leading edge of a moving wave and is often considered to be a wave crest rather than a trough. You can make waves in all shapes but plane waves and circular waves are easiest to create and study (Figure 23.15). The crests of a plane wave look like parallel lines. The crests of a circular wave are circles. A plane wave can be started by disturbing water in a line. A circular wave can be started by disturbing water at a single point. The shape of the wave front determines the direction the wave moves. Circular waves have circular wave fronts that move outward from the center. Plane waves have straight wave fronts that move in a line perpendicular to the wave fronts. Both circular and plane waves eventually hit surfaces. Four interactions are possible when a wave encounters a surface reflection, refraction, diffraction, or absorption. wave front - the leading edge of a moving wave. plane wave - moving waves that have crests in parallel straight lines. circular wave - moving waves that have crests that form circles around a single point where the wave began. Figure 23.15: Plane waves move perpendicular to the wave fronts. Circular waves radiate outward from a single point. 567

2 WAVES Wave interactions Boundaries Reflection Refraction Diffraction Absorption A boundary is an edge or surface where one material meets a different material. The surface of a glass window is a boundary. A wave traveling in the air experiences a sudden change when it encounters the boundary between the air and the glass of a window. Reflection, refraction, and diffraction usually occur at boundaries. Absorption also occurs at a boundary, but happens to a greater extent within the body of a material. When a wave bounces off an object we call it reflection. A reflected wave is like the original wave but moving in a new direction. The wavelength and frequency are usually unchanged. An echo is an example of a sound wave reflecting from a distant object or wall. People who design concert halls pay careful attention to the reflection of sound from the walls and ceiling. Refraction occurs when a wave bends as it crosses a boundary. We say the wave is refracted as it passes through the boundary. The process of refraction of light through eyeglasses helps people see better. The lenses in a pair of glasses bend incoming light waves so that an image is correctly focused within the eye. The process of a wave bending around a corner or passing through an opening is called diffraction. We say a wave is diffracted when it is changed by passing through a hole or around an edge. Diffraction usually changes the direction and shape of the wave. When a plane wave passes through a small hole, diffraction turns it into a circular wave (Figure 23.16). Diffraction explains why you can hear sound through a partially closed door. Diffraction causes the sound wave to spread out from any small opening. Absorption is what happens when the amplitude of a wave gets smaller and smaller as it passes through a material. The wave energy is transferred to the absorbing material. A sponge can absorb a water wave while letting the water pass. Theaters often use heavy curtains to absorb sound waves so the audience cannot hear backstage noise. The tinted glass or plastic in the lenses of your sunglasses absorbs some of the energy in light waves. Cutting down the energy of light makes your vision more comfortable on a bright, sunny day so you don t have to squint! reflection - the process of a wave bouncing off an object. refraction - the process of a wave bending as it crosses a boundary between two materials. diffraction - the process of a wave bending around a corner or passing through an opening. absorption - what happens when the amplitude of a wave gets smaller and smaller as it passes through a material. Figure 23.16: An illustration of diffraction. 568 UNIT 8 WAVES, SOUND, AND LIGHT

3 Transverse and longitudinal waves Wave pulses Transverse waves A wave pulse is a short burst of a traveling wave. A pulse can be produced with a single up-down movement. The illustrations below show wave pulses in springs. You can see the difference between the two basic kinds of waves transverse and longitudinal by observing the motion of a wave pulse. The oscillations of a transverse wave are not in the direction the wave moves. For example, the wave pulse in the illustration below moves from left to right. The oscillation (caused by the boy s hand) is up and down. Water waves are an example of a transverse wave (Figure top). p transverse wave - a wave is transverse if its oscillations are not in the direction it moves. longitudinal wave - a wave is longitudinal if its oscillations are in the direction it moves. Longitudinal waves The oscillations of a longitudinal wave are in the same direction that the wave moves (Figure bottom). A sharp push-pull on the end of the spring makes a traveling wave pulse as portions of the spring compress then relax. The direction of the compressions are in the same direction that the wave moves. Sound waves are longitudinal waves. Figure 23.17: Transverse and longitudinal waves. 569

4 WAVES Constructive and destructive interference Wave pulses If you have a long elastic string attached to a wall, you can make a wave pulse. First you place the free end of the string over the back of a chair. The string should be straight so that each part of it is in a neutral position. To make the pulse, you pull down a short length of the string behind the chair and let go. The pulse then races away from the chair all the way to the wall. You can see the wave pulse move on the string. Each section of string experiences the pulse and returns to the neutral position after the wave pulse has moved past it. constructive interference - when waves add up to make a larger amplitude. destructive interference - when waves add up to make a smaller, or zero, amplitude. Constructive interference Destructive interference Suppose you make two wave pulses on a stretched string. One comes from the left and the other comes from the right. When the waves meet, they combine to make a single large pulse. Constructive interference happens when waves combine to make a larger amplitude (Figure 23.18). There is another way to add two pulses. Sometimes one pulse is on top of the string and the other is on the bottom. When these pulses meet in the middle, they cancel each other out (Figure 23.19). One pulse pulls the string up and the other pulls it down. The result is that the string flattens and both pulses vanish for a moment. In destructive interference, waves add up to make a wave with smaller or zero amplitude. After interfering, both wave pulses separate again and travel on their own. This is surprising if you think about it. For a moment, the middle of the cord is flat, but a moment later, two wave pulses come out of the flat part and race away from each other. Waves still store energy, even during destructive interference. Noise cancelling headphones are based on technology that uses destructive interference. Figure 23.18: This is an example of constructive interference. Figure 23.19: This is an example of destructive interference. 570 UNIT 8 WAVES, SOUND, AND LIGHT

5 23.3 Section Review 1. How does the motion of a plane wave differ from the motion of a circular wave? 2. For each of the examples below, identify whether reflection, refraction, diffraction, or absorption is happening. a. During a total solar eclipse, the Moon is in front of the Sun but you can still see some sunlight around the edges of the Moon. b. The black surface of a parking lot gets hot in the summer when exposed to sunlight. c. The image at the right of a straw in a glass looks funny. d. When you look in a mirror, you can see yourself. e. Sound seems muffled when it is occurring on the other side of a wall. f. Light waves bend when they move from water to air. g. A ball bounces back when you throw it at a wall. 3. When a wave is being absorbed, what happens to the amplitude of the wave? Use the term energy in your explanation. 4. Compare and contrast transverse waves and longitudinal waves. 5. Two waves combine to make a wave that is larger than either wave by itself. Is this constructive or destructive interference? 6. When constructive interference happens between two sound waves, the sound will get louder. What does this tell you about the relationship between amplitude and volume of sound? 7. One wave on a string is moving toward the right and another is moving toward the left. When, they meet in the middle, half of the cycle of the wave from the right overlaps with half of the cycle of the wave from the left. The result is that the string gets flat when the two waves meet. What happened? What will happen after the waves meet? TECHNOLOGY Noise-Cancelling Headphones The graphic below illustrates how noise-cancelling headphones work. Study the graphic and write a description that explains why noisecancelling technology is a good way to reduce noise. Verify your description by doing some research about these special headphones. 571