Waves PDF

What are waves? When a duck lands in a pond, it disturbs the ▶ Main Idea water. This disturbance produces a series of circles Waves transfer sound that grow larger as they mo...

What are waves? When a duck lands in a pond, it disturbs the ▶ Main Idea water. This disturbance produces a series of circles Waves transfer sound that grow larger as they move away from the duck. energy from a source These circles are waves. A wave is a disturbance that through a medium. transfers energy from one point to another. Some ▶ Vocabulary waves, such as light waves, can travel through empty transverse wave, p. 646 space. Other kinds, such as sound waves, must compressional wave, p. 646 travel through a medium, or substance, in order to compression, p. 647 transfer energy. A medium can be a solid, a liquid, rarefaction, p. 647 or a gas. The movement of particles by a wave is wavelength, p. 648 called vibration. Waves are classified by the type of frequency, p. 648 vibration they cause in a medium. amplitude, p. 648 reflection, p. 650 Types of Waves refraction, p. 651 When a transverse wave travels through a Doppler effect, p. 652 medium, matter moves up and down as the wave travels through it. A wave that you might make by -Glossary moving in the water is a transverse wave. at www.macmillanmh.com When a compressional wave travels, matter ▶ Reading Skill moves back and forth as the wave travels through it. Compare and Contrast Think of a long coiled spring. If you move one part Different Alike Different back and forth, it hits the next coil on the spring. The coil then returns in the other direction. A wave travels down the coils, moving each coil back and forth. A feeding dunlin causes ripples in still water. ▼ ▶ Technology Explore solar radiation with an engineer. 646 EXPLAIN  Types of Waves The vibration moves up and down. Transverse Wave Particles of the medium move up and down. crest wavelength amplitude trough The waves are moving in this direction. Compressional Wave The vibration moves left and right. Particles of the medium move left wavelength and right. rarefaction compression The vibration of the coils produces All waves travel and transfer energy a compression, an area where from one point to another, with little particles are pushed together. Behind or no displacement of the particles the compression is a rarefaction of the medium. Once the wave has (rayr uh FAK shuhn), an area where passed, the particles end up in about particles are spread apart. The wave the same position they started in. This moves through its medium as a series is similar to “the wave” at a sporting of compressions and rarefactions. event. After the wave moves across a group of spectators, the fans are once Features of Waves again sitting in the same places they The crest of a transverse wave is were in before the wave. its highest point. In a compressional Quick Check wave, the crest is the point of greatest compression—the area where particles Compare and Contrast How are are closest together. Waves also have light energy and sound energy troughs (trawfs). The trough of a similar? How are they different? transverse wave is its lowest point. In a Critical Thinking Explain which compressional wave, the trough is the type of wave—transverse or point of greatest rarefaction—the area compressional—is most similar to where particles are farthest apart. “the wave” at a sporting event. 647 EXPLAIN How can you measure waves? Wavelength is the distance between wave crests or troughs. Frequency is a measure of how many wave crests or troughs pass a given point in one unit of time, such as a second. High-frequency waves have shorter wavelengths and transfer greater energy. If you vibrated a clothesline rapidly, the waves would move at a high frequency. If you vibrated it more slowly, the waves would have a lower frequency. The period of a wave is the amount of time it takes for a wave to complete one full cycle. Period is the inverse of frequency. Amplitude, the height of the wave from its trough or crest to its midpoint, is a measure of the wave’s intensity. In the ocean, a wave’s amplitude increases as it nears the shore. Frequency and Speed of Waves Frequency is measured in hertz (Hz), the number of waves per second. Hertz means “cycles per second” with respect to wavelength frequency. Picture a motorboat passing a floating buoy. Waves from the boat cause the buoy to vibrate up and down 4 times in 16 seconds. The frequency of the vibrations is 0.25 hertz. Frequency and Amplitude Frequency lower Lower-frequency wavelength waves disturb a medium with fewer wave cycles higher per second than higher-frequency wavelength waves would. 1 second Amplitude amplitude Higher-amplitude lower waves are more intense, or amplitude stronger, than lower-amplitude waves. higher 648 EXPLAIN  Wave Motion Read a Diagram Why does the pelican stay in the same place? Clue: Look at how the wave moves. Watch waves at www.macmillanmh.com Speed describes how fast something travels in a specific amount of time. move fastest through solids, slower The distance a wave travels per second through liquids, and slowest through determines its speed. For example, a gases. Think of two springs of equal boat passes within 65 meters of a buoy. length, one with more coils than the The waves from the boat take 5 seconds other. The coils represent particles of to reach the buoy. The wave’s speed is matter in the medium. The spring with 65 meters divided by 5 seconds, or 13 more coils conducts sound waves more meters per second. A high-frequency rapidly, because the particles are in wave can travel slowly. Likewise, a fast- closer contact and will spring back moving wave can have a low frequency. and transmit energy faster. Factors That Affect Speed Quick Check The medium through which a wave Compare and Contrast How is travels affects its speed. The depth of a wave’s frequency different from the water affects the speed of ocean its speed? waves. The deeper the water, the faster the wave travels. In sound waves, the Critical Thinking Which would distance between particles affects wave transmit sound better: a wire or air? speed. Sound waves, as you might infer, Explain. 649 EXPLAIN How does sound travel? Sonar A sound wave is a compressional wave produced by vibrations in matter. Molecules in the medium move back and forth, pushing nearby molecules. A compression forms as the molecules are pushed closer together. A rarefaction then forms behind the compression. The compression and the rarefaction together form one cycle of a sound wave. Since sound waves depend on the compression of matter, they need a medium to travel through. For this reason, sound waves cannot travel through empty space. Tuning forks vibrate at particular frequencies. The vibrations produce compression waves in the air. Waves can be studied by scientists with devices Reflected Sound such as oscilloscopes (uh SIL uh skohps). When you look in a mirror, light waves bounce off the mirror into Speed of Sound your eyes and enable you to see your image. Sounds also reflect off objects. Reflection refers to how waves bounce off an object and change their direction of travel. An echo is a reflected sound wave. Speed Medium (in meters The technology called sonar uses per second) reflected sound waves, or echoes, to air 334 fresh water 1,461 locate unseen objects and to make maps seawater 1,490 of the ocean floor. Fishers and marine silver 2,610 biologists use sonar to locate schools of glass 5,000 fish. Sonar works by bouncing sound waves off an object and measuring the time the echoes take to return. The Read a Table speed of sound waves in a particular What happens to the speed medium is known for many different of the sound of thunder as it substances, such as seawater and air. moves from the air to the lake? Therefore, the exact location of an Clue: Does sound move faster object can be determined from the in air or water? travel time of the echo in a medium. 650 EXPLAIN  String Telephone Make a Model Obtain two paper cups and about 10 m of string. Sound waves reflected from the Make a small hole in the bottom ocean floor provide detailed of each cup. Thread one end of the information about its contours. string through each hole. Tie a knot 1 The sonar device sends a in each end of the string so the signal into the water. ends cannot slip through the holes. Experiment Try your model with 2 The signal reflects off an a partner. Each partner should object on the ocean floor. take one of the cups. Move far enough apart that the string is taut 3 The signal’s direction between the two of you. and travel time are used to determine the object’s location. Observe Take turns speaking softly into the cup as your partner listens. How well are you able to communicate? Refracted Sound Use Variables Coat the string with wax. Does doing this improve your Refraction occurs when the direction ability to communicate using this of a wave changes because of a change device? in medium. This happens because Predict What other variables could waves move at different speeds through you test that might make your different media. Sound waves travel string telephone more effective? faster through deep water than through shallow water. Sound waves traveling through air are refracted as they enter water. If the type of medium through which waves travel changes, the waves generally change speed and direction. Absorbing Sound Waves The material a sound wave strikes affects how the sound wave moves. Hard surfaces easily reflect sound Quick Check waves. Soft surfaces absorb sound Compare and Contrast How are waves. A material that absorbs sound reflection and refraction similar? well does not reflect sound waves. How are they different? Ceiling tiles that absorb sound waves are used in concert halls, offices, and Critical Thinking How could you libraries. increase echoes in a room? 651 EXPLAIN What are properties of sounds? When the water in a whistling Certain whistles produce sounds with teakettle boils, you hear a high, shrill frequencies greater than 20,000 hertz. sound. When a musician plays a If you used this type of whistle, you tuba, you hear a low, deep sound. would not hear anything, but most Although both sounds are made by dogs would hear it clearly. Bats use waves, they differ in pitch. The pitch, higher-frequency sounds as sonar to or the highness or lowness of a sound, find their insect prey. Other animals, depends on the frequency of the sound such as elephants, can hear lower- waves. High-pitched sounds have a frequency sounds. They communicate high frequency. Low-pitched sounds over long distances using sounds that have a low frequency. The teakettle are below the range of human hearing. makes a sound with a higher pitch. The tuba makes a sound with a lower pitch. The Doppler Effect Most people can hear sound waves The pitch of a sound can seem to in the range of about 20 hertz to about change if its source or listener is in 20,000 hertz. These are sound waves motion. This is called the Doppler that cause matter to vibrate between effect. Think of what happens when 20 times and 20,000 times per second. the driver of an approaching car blows Some animals can hear sounds with the car’s horn. The motion of the car frequencies higher or lower than our toward you causes the sound waves in ears can detect. For example, many front of the vehicle to arrive closer and dogs have the ability to hear sounds closer to one another. This increases with frequencies of up to 50,000 hertz. the frequency of the sound waves. The designs and materials of instruments produce sounds of various pitches. 652 EXPLAIN Therefore, you hear a sound wave with a higher pitch than the wave produced by the car’s horn. The opposite occurs as the car passes. Sound waves behind the receding car arrive farther and farther apart. Your ears hear a sound wave with a lower pitch. Although you heard the pitch of the horn move up and down, the person inside the passing car would not hear any change. The change in pitch you heard only had to do with the way in which you and the source of the sound were moving in relation to each other. Hearing protection is necessary for airport ground-crew workers due to Volume of a Sound high-decibel sounds. The difference in the loudness of a sound is called volume. The amount of energy, or intensity, of the sound wave Interference can be either positive determines the volume of a sound. or negative. If the crests or troughs of Volume is determined by amplitude. the waves meet, as they would if both The larger the amplitude of a wave, stereos were side by side and playing the greater its energy. Loud sounds are the same song, the amplitudes of the produced by high-intensity waves with waves combine. The combined sound large amplitudes. waves of the stereos would produce a The volume of a sound is measured louder sound than that from one stereo in units called decibels (dB). A whisper alone. This is constructive interference. has a volume of about 30 decibels, and However, this does not occur if the regular speech has a volume of about crest of one wave meets the trough of 60 decibels. Sounds of greater than 90 another. In this case the sound waves decibels can damage people’s hearing. together have a lower amplitude than the sound made by one source alone. Interference This is destructive interference. If you play a song on a stereo, sound waves move through the air. Quick Check Listeners can hear the sound produced. Compare and Contrast How Suppose another set of speakers played do destructive interference and the same song in the same room. This constructive interference compare? motion of two or more waves passing through the same medium at the same Critical Thinking How are sound time is called interference. volume and amplitude related? 653 EXPLAIN How do we hear music? The sounds made by musical “Music” is a combination of sounds instruments differ according to the that a listener finds pleasing, and types of vibrations they produce. “noise” is a combination of sounds Stringed instruments, such as guitars that a listener finds unpleasant. Music or violins, produce vibrations of wires usually blends instrumental or vocal or strings. Wind and brass instruments, tones in a structured and continuous such as trombones or flutes, produce manner. However, any agreeable and vibrations in columns of air. The harmonious sounds can be interpreted difference among sounds of the as musical. Sound that is interpreted same pitch and amplitude in various as musical has a mathematical instruments is called sound quality. structure of both tones and silence. As sound waves travel, they transfer This structure is often referred to as energy. When sound waves reach your rhythm. Sounds that lack harmony, ear, they pass through the ear canal rhythm, and mathematical structure to your eardrum. The sound waves are interpreted as noise. strike the eardrum and cause it to vibrate. The vibrations stimulate nerve Quick Check cells located deep inside the ear. These Compare and Contrast How are vibrations are then converted to nerve noise and music similar? How are impulses that your brain recognizes. By they different? interpreting pitch and sound quality, the brain identifies different sounds. Critical Thinking What must occur for you to hear a sound? Human Ear middle outer ear ear inner ear Sounds cause the air hammer anvil around your ear to vibrate. Your outer eardrum ear directs these vibrations down the ear canal. auditory nerve cochlea ear canal stirrup Eustachian tube 654 EXPLAIN