Unit 8 : Waves: Sound and Light (Chp 25-27)

1. Unit 8: Waves: Sound and Light (Chp 25-27) Wavestransmitenergy through space and time.

2. (source of all waves) vibration: repeating back-and-forth motionfrom an equilibrium position. wave: disturbancethat transmits energy through space with no transport of matter. Light and sound are both forms of energy that move through space as waves.

3. 25.2Wave Description A weight on a spring shows simple harmonic motion. A marking pen attached to the bob traces a sine curve on a moving sheet of paper. A sine curve is a pictorial representation of a wave. The Parts of a Wave

4. 25.2Wave Description amplitude: wave height from zero tocrest wavelength: distancebetween two crests frequency: number of waves per unit of time

5. 25.2Wave Description (0.000000001 m) wavelength () meters (m) or nanometers (nm) frequency () Hertz (Hz),s–1,1/s,per sec

6. 25.2Wave Description • Higher frequencies are measured in • kilohertz (kHz—thousands of hertz) • megahertz (MHz—millions of hertz) • gigahertz (GHz—billions of hertz) Electrons in the antenna of a 960 AM station vibrate 960,000 times per sec (960 kHz radio waves) 103.3 FM vibrates at 103.3 MHz or 103,300,000 waves per sec.

7. 25.3Wave Motion wave: disturbancethat transmits energy through space with no transport of matter. Think about the very simple wave produced when one end of a stretched string is shaken up and down. Each part of the string moves up and down and the disturbance moves along the length of the string. The disturbance moves, not parts of the string itself.

8. 25.3Wave Motion Drop a stone in a pond and it is the disturbance that moves, not the water. When someone speaks to you from across the room, the sound wave is a disturbance in the air that travels across the room. The air molecules themselves do not move along. The air, like the rope and the water, is the mediumthrough which wave energy travels.

9. 25.5 – 25.6Transverse & Longitudinal Waves Suppose you create a wave along a rope by shaking the free end up and down. The motion of the rope is at right angles to the direction in which the wave is moving. transverse wave: motion of the medium is perpendicular to the direction in which a wave travels.

10. 25.5 – 25.6Transverse & Longitudinal Waves Sometimes the particles of the medium move back and forth in the same direction in which the wave travels. longitudinal wave: particles of the medium vibrateparallelto or along the direction of the wave.

11. 25.5 – 25.6Transverse & Longitudinal Waves • Both waves can be shown with a loosely coiled spring. • Shake up and down for a transverse wave.

12. 25.5 – 25.6Transverse & Longitudinal Waves • Both waves can be shown with a loosely coiled spring. • Shake up and down for a transverse wave. • Shake in and out for a longitudinal wave.

13. Quick Quiz! • What do waves transfer? • amplitude • wavelength • frequency • energy

14. Quick Quiz. • The vibrations along a transverse wave move in a direction… • along the wave in the same direction. • perpendicular to the wave. • parallel to the wave. • along the wave in the opposite direction.

15. Quick Quiz. • The vibrations along a longitudinal wave move in a direction • along and parallel to the wave. • perpendicular to the wave. • below the wave. • above the wave.

16. 25.7Interference If you drop two rocks in water, the waves produced by each can overlap and form an interference pattern. interference: (common to ALL waves) pattern from waves meeting causing wave effects to be increased, decreased, or cancelled.

17. 25.7Interference constructive interference: (in phase) crestoverlaps another crestreinforcing their effects increasingamplitude destructive interference: (out of phase) crestoverlaps a troughcanceling their effects to zeroamplitude (no wave)

18. 25.7Interference Overlapping concentric circles produce an interference pattern. • Two overlapping water waves produce an interference pattern. animation

19. 25.9The Doppler Effect If a bug bobs in water at a constant frequency, the wavelength is the same for waves in all directions. The wave frequency is the same as the bug’s bobbing frequency.

20. 25.9The Doppler Effect The bug has the same bobbing frequency as before. But, observer B observes a higher frequency if the bug is moving toward the observer. Observer A observes a lower frequency if the bug is moving away from the observer.

21. 25.9The Doppler Effect Doppler effect: apparent change in frequencydue to the motion of a source (or receiver). The greater the speed of the source, the greater will be the Doppler effect.

22. 25.9The Doppler Effect (Sound) The Doppler effect causes the changing pitch of a siren. When a fire truck approaches, the pitch sounds higher than normal because the sound wavecrests arrive more frequently. When the fire truck passes and moves away, you hear a drop in pitch because the wave crests are arriving less frequently.

23. 25.9The Doppler Effect (Sound) Police bounce them off moving cars. A computer built into the radar system compares the frequency of the radar with the frequency of the reflected waves to find the speed of the car.

24. 25.9The Doppler Effect (Light) • The Doppler effect also occurs for light. • When a light source approaches, the measured frequencyincreasescalled a blue shift,toward the high-frequency, or blue, end of the spectrum. • When it recedes, the frequencydecreases called a red shift, referring toward the low-frequency, or red, end of the color spectrum.

25. 25.9The Doppler Effect Measurement of red shift enables astronomers to analyze distant galaxies.

26. 25.10 – 25.11Bow Waves & Shock Waves v = speed of bug vw = wave speed The crests overlap at the edges, and the pattern made by these overlapping crests is a V shape, called a bow wave.

27. 25.10 – 25.11Bow Waves & Shock Waves A shock waveis a three-dimensional wave that consists of overlapping spheres that form a cone. The sharp crack heard when the shock wave reaches the listeners is called a sonic boom.

28. 25.10 – 25.11Bow Waves & Shock Waves Sound wave crests normally reach our ears one at a time and are perceived as a continuous tone. But when a craft moves faster than sound, the crests overlap and encounter the listener in a single burst called a sonic boom.

29. Quick Quiz! • Interference is characteristic of… • sound waves. • light waves. • water waves. • all waves.

30. Quick Quiz. • The Doppler effect changes the… • frequency due to motion. • speed of sound due to motion. • speed of light due to motion. • radar waves in a police car.

31. Quick Quiz. • Which would cause an increase in wave frequency? • moving away from the wave source • wave source moving away from you • moving toward a wave source • wave source moving toward you

32. Quick Quiz. • Shock waves are produced by waves of sound… • constructively interfering. • destructively interfering. • moving faster than the source producing them. • that never overlap.

33. The vocal chords in the singer’s throat vibrate, causing air molecules to vibrate. A series of ripples travel as longitudinal waves through the air. Vibrations in the eardrum send rhythmic electrical impulses into your brain and you hear the voice of the singer. SOUND

34. 26.2Sound in Air Clap your hands and you produce a sound pulse that goes out in all directions. Each particle moves back and forth along the direction of motion of the expanding wave. A compression travels along the spring similar to the way a sound wave travels in air.

35. 26.2Sound in Air When you open a door, the door pushes the molecules next to it into their neighbors. Neighboring molecules then push into their neighbors, and so on, like a compression wave moving along a spring. compression: pulse of compressedair

36. 26.2Sound in Air When you close the door, the door pushes neighboring air molecules out of the room. This produces an area of low pressure next to the door. Nearby molecules move in, leaving a zone of lower pressure behind them. rarefaction: pulse of low-pressure air

37. 26.2Sound in Air For all wave motion, it is not the medium that travels across the room, but a pulse that travels. • Consider sound waves in a tube. • As a tuning fork vibrates, a series of compressions and rarefactions travels down the tube.

38. 26.3Media That Transmit Sound Most sounds you hear are transmitted through the air. Put your ear to a metal fence and have a friend tap it far away. Sound is transmitted louder and faster by the metal than by the air. Click two rocks together underwater while your ear is submerged. You’ll hear the clicking sound very clearly. In general, soundtravelsfaster in liquids than in gases, and still faster in solids.

39. 26.3Media That Transmit Sound Sound cannot travel in a vacuum. Transmission of sound requires a medium. There may be vibrations, but if there is nothing to compress and expand, there can be no sound. Sound can be heard from the ringing bell when air is inside the jar, but not when the air is removed.

40. 26.4Speed of Sound If you watch a distant person hammering, the sound of the blow takes time to reach your ears, so you see the blow and then hear it. You hear thunder after you see the lightning. These experiences are evidence that sound is much slower than light.

41. 26.4Speed of Sound The speed of sound in air at 0°C is about 330 m/s (about 740 mi/hr). (this is about one-millionth the speed of light) Higher temperaturesincrease this speed slightly—faster-moving molecules bump into each other more often. Sound travels about 4 times faster in water than in air, and about 15 times faster in steel than in air.

42. 26.4Speed of Sound think! How far away is a storm if you note a 3 sec delay between a lightning flash and the sound of thunder? (speed of sound in air of 340 m/s) Answer: (340 m/s) × (3 s) = about 1000 m or 1 km. Time for the light is negligible, so the storm is about 1 km away.

43. Quick Quiz! • Sound travels in air by a series of… • bursts and pitches. • pressures and forces. • compressions and rarefactions. • crests and troughs.

44. Quick Quiz. • Sound travels faster in… • a vacuum compared to liquids. • gases compared to liquids. • gases compared to solids. • solids compared to gases.

45. Quick Quiz. • The speed of sound varies with… • amplitude. • frequency. • temperature. • pitch.

46. 26.5Loudness The intensity of a sound is proportional to the square of the amplitude of a sound wave and is measured by instruments. But loudness differs for different people. It’s a physiological sensation sensed in the brain, but it is related to sound intensity. The unit of intensity for sound is the decibel (dB), after Alexander Graham Bell, inventor of the telephone.

47. 26.5Loudness • Starting with zero at the threshold normal hearing, an increase of each 10 dB means that sound intensity increases by a factor of 10. • A sound of 10 dB is 10 times as intense as sound of 0 dB. • 20 dB is not twice but 10 times as intense as 10 dB, or 100 times as intense as the threshold of hearing. • A 60-dB sound is 100 times as intense as a 40-dB sound.

48. 26.5Loudness Hearing damage begins at 85 decibels. A single burst of sound can produce vibrations intense enough to tear apart the organ of Corti, the receptor organ in the inner ear.

49. 26.8Resonance video clip “Tacoma Narrows Bridge” resonance: when the frequency of a forcedvibrationmatches an object’s natural frequency it dramaticallyincreases the amplitude. If two tuning forks of same frequency, striking one fork sets the other fork into vibration. The frequency of these pushes matches the natural frequency of the fork, so the pushes increase the amplitude of the fork’s vibration.

50. 26.8Resonance • The first compression gives the fork a tiny push. • The fork bends. • The fork returns to its initial position. • It overshoots in the opposite direction. • When it returns to its initial position, the next compression arrives to repeat the cycle.