Presentation on theme: "1 Wave Motion ( 波動 ) Wave motion is a kind of vibration(oscillation), through which, energy (not matter) can be transferred from one place to other places."— Presentation transcript:
1 Wave Motion ( 波動 ) Wave motion is a kind of vibration(oscillation), through which, energy (not matter) can be transferred from one place to other places.
2 Vibration of water molecules perpendicular to wave(energy) travelling direction.
3 Why can ocean waves lift a ship? Waves carry energy
4 Tsunamis carry a huge amount of energy
5 2 natures of wave. 1.Can travel through vacuum called EM (electromagnetic)waves. 2.Travel through medium called mechanical waves. e.g of EM waves: Radio wave Microwave Infrared radiation Visible light Ultraviolet radiation X rays Gamma rays Cosmic rays e.g. of mechanical waves: Sound wave Water wave Vibrating string Vibrating spring
7 transverse wave( 横波 ) 、 longitudinal wave( 縱波 ) 1.transverse wave : vibration is perpendicular to wave (energy) traveling direction 2 types of wave vibration Simulation of transverse wave: CH 10
8 vibration of particles light Some e.g.
10 Light(a kind of electromagnet wave( 電磁波 )is a transverse wave 1.Vibration of electric field & magnetic field 2.Electric field & magnetic field is a kind of energy. 3.Light is a form of energy.
11 2. Longitudinal wave : vibration is parallel to wave traveling direction. rarefaction Compression Simulation of longitudinal wave: CH 10
12 1.Sound is a longitudinal wave. 2.Sound cannot travel through a vacuum. 3.Through air, sound waves travel at 330 m/s. 4.Sound waves move faster through solid and liquid than through gases. 5.Audiable frequency: 20 –20000 Hz 6.f > Hz called ultrasonic wave 7.Ultrasonic wave is used to detect flaws in metal pipe; to locate shoals of fish; to examine an unborn baby & as cleaners. 8.The quietest sound = 0 dB (decibels, unit of sound intensity) 9.Person talking = 60 dB 10.annoying sounds > 100 dB 11.Threshold of feeling = 120 dB Sound
13 To prove that sound is a longitudinal wave.
14 1.Amplitude: maximum displacement of particles from equilibrium position. 2. The bigger the amplitude, the greater the energy transferred Simulation: CH 10
15 Frequency (f/Hz): number of vibrations in 1 second (Note: f always keeps constant) Wavelength(λ/m): distance between 2 consecutive particles that are in phase (distance between 2 crests/compression or 2 troughs/rarefaction) Simulation : CH 10
16 Wave (energy) traveling speed(ms -1 ): 1.V = f λ 2.Sound speed in solid > in liquid > in gas 3.Water wave travels faster(slower) in deeper (shallower) region Period (T/s): Time for 1 vibration T = 1 / f
V = f λ=(2)(100) = 200 ms -1 (= 720 km h -1 ) V = distance / time t = 1500 / 720 = 2 hours
19 Phase ( 相位 )Phase ( 相位 ) 。 In phase In phase ( 同相 ) : vibrations that are in stepsin steps anti- phaseanti- phase ( 反相 ) : vibrations that are exactly opposite to each other. Wavelength: Distance between 2 consecutive particles that are in phase = λ Simulation: CH 10 transverse wave
20 at rest 37.5 cm 12.5 cm The above diagram shows the shape of a slinky spring at 0.5 s after the vibration is started, 1.Indicate the positions of compression (C) and rarefaction ( R ). 2.Is the vibration started with a push or a pull ？ push 3.Find λ = ？ 37.5÷2.5 = 15 cm(Note: careful in choosing C & R) 4.Find the speed and frequency of the wave. ( )÷0.5 = 100 cm/s f = V ÷ λ = 100 ÷ 15 = 6.7 Hz
25 If M Richter magnitude scale ( 黎克特制 )is increased by 1, E ( 能量 )is magnified by a factor of (=~32). In other words, the seismic ( 地 震 )energy of a M=6 earthquake is about 32 times as large as that of an earthquake M=5 earthquake, and is ~1000 ( =32x32= 1024)times that of an M=4 earthquake. Energy released by M = 8 is greater than that by M = 1 by : ( 32x32x32x32x32x32x32 = ~ ~ 300 億倍能量 Earth's daily receipt of solar energy ~ M = 12
26 Richter Effect Less than 3.5 not felt, but recorded Often felt, but rarely causes damage. Under 6.0 At most slight damage to well- designed buildings. Cause major damage to poorly constructed buildings over small regions Can be destructive in areas up to about 100 kilometers across where people live Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Cause serious damage in areas several hundred kilometers across.
27 As if a 2 km rocky meteorite impacting to our earth at 90,000 km/h M = 10 : The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude of 9.5 Never recorded
28 e.g 1 If the speed of a wave on water surface is 8 m/s and theλis 80m 。 Find the time between the arrival of 2 successive waves ？ 10 s e.g 2 If the frequency of a wave is 10Hz and λ = 33 m 。 1.Find the wave speed = ？ 330 m/s 2.If the frequency is doubled, find the new λ = ？ 16.5 m
29 Period 週期 (T/s) Time for 1 complete vibration 。 T = 1/ f or: Time for wave to travel 1λ. t=1T t=2T t=0
30 eg The following diagram shows a transverse wave traveling from left to right, If each particle can finish 4 vibrations in 16 s: P Q S R T 5cm 2cm (a)Find the amplitude 5 cm (b) find the wave speed 0.02 m/s (c) Sketch the shape of the diagram after half a period.
33 Describing waves with graphs: 1.Displacement – distance graph of a wave 2.Displacement – time graph of a wave
34 1.Displacement – distance graph of a wave shows the displacements (y-axis) of particles at various positions (x-axis) of a wave from their equilibrium position at a particular time, it can show the amplitude and wavelength of the wave. Displacement/m Position/m + - P.23
35 Use displacement – distance graph to determine the motion of particles displacement / cm distance / cm 1 –1– P Q R S direction of travel later present P : moving downwards R : moving upwards Q : momentarily at rest. S : momentarily at rest. Traveling direction must be given
36 The diagram below shows the displacement - distance graph for a transverse wave with amplitude 1.0 cm moving to the right at 8 cm s
37 2. Displacement – time graph of a wave shows the displacement (y-axis) of a particular particle from its equilibrium position with time (x-axis), it shows only amplitude and period(or frequency) of the wave. The displacement - time graph for particle 13 is shown below. 1 period
38 Traveling longitudinal wave (P.23): Traveling direction must be given
39 Displacement-distance graph of longitudinal waves(P.27): The Displacement-distance graph of longitudinal waves looks like a transverse wave!
40 Class work: P.33 Questions: 1-8 HW: P.34 7,8,9,10,11
41 2. Displacement – time graph of a wave Shows the displacements (y-axis) of a particular particle at various time (x-axis) of a wave from its equilibrium position, it can show the amplitude frequency and period of the wave. displacement / cm Time/s T amplitude period 2T2T 1 0 –1–1
42 P P.28 4,5,6,7
48 E F G H I J K L MA B C D N O 5 cm E F GH I JK L MA B CD N O At time = 0 At time = t A longitudinal wave is traveling to the right, some of the the medium particles are recorded as: Take the displacement to the right as positive. Sketch the displacement-distance graph of the wave.
49 displacement / cm distance 6 66 A G M D J Time = t 3 s later Answer: If the frequency of the wave is 0.5 Hz, on the displacement-distance above, sketch the displacement-distance graph of the wave (from coil A to coil O) 3 s later
50 4 0 44 distance / cm displacement / cm travelling direction A BC D A transverse wave is traveling to the right shown: At the instant shown, which of the particles A, B, C or D, is/are (i)momentarily at rest, A (ii)moving upwards, and B, (iii)moving downwards? C, D If particle B performs 5 complete oscillations in 2 s, Sketch the displacement-time graph of particle A from t = 0 s to t = 0.4 s.
displacement / cm A 44 time / s
52 e.g. P26 check point 1 – 4 HW P28 4,5,6,7
54 Properties of Waves : 1.Reflection 2.Refraction 3.Diffraction 4.Interference Use water waves to demonstrate wave properties.
55 Water waves work as lens. Ripple tank crest:converge light rays water Trough : diverge light rays Ripple tank ( 水波槽原理 ) brightdark
56 Wavefront( 波陣面 ):a line or surface over which all particles are vibrating at the same phase Wave travelling direction Circular wavefront wavefront Straight wavefront WavefrontRays
57 Use water waves to demonstrate 4 properties of wave ： 1.reflection 2.refraction 3.diffraction 4.interference Straight wave Circular wave 1. reflection:
58 Law of reflection: incident angle = reflection angle i = r Incident ray Normal i r Reflected ray
60 P.55 check point 1,2, 3,
61 Phase change in refraction P.57 check point 1-3
62 2. refraction: When wave travels from one medium to another, its speed will change, this leads to refraction. Straight waveStraight wave 1 Straight wave Straight wave 2 shallower deeper
63 Travelling direction > 2 decreases f unchanged v decreases
64 Shallower: 1. decreases 2.V decreases 3.rays bend towards normal normal
65 They converge to a point (focus). shallow convex region:
66 They diverge from a point (focus). shallow concave region:
67 P.55 1,2,3,4
70 P 75 (5)
71 3. diffraction Diffraction is the bending of waves around obstacles. Diffraction around an edge:
72 increases, degree of diffraction increases. Degree of diffraction
73 Diffraction through a slit The degree of diffraction increases if: 2. the slit width decreases 1. increases
74 e.g. of diffraction: 1.Sound waves bend around the rim of loudspeakers 2.TV broadcasting: TV antennas must point directly to transmitting station.
75 P Decrease the depth of water 2.Increase the width of the slit
76 radio station hill house Radio signals are broadcast in radio waves with wavelengths ranging from a few metres to a few kilometres as shown belows: (a)Explain, with the help of a diagram, how the antenna of the house receives the signals from the radio station. (2 marks) (b)Of which wavelength, long or short, would the antenna have a better reception? Explain briefly. (2 marks)
77 (a) (diffraction shown) The waves diffract round the hill and reach the antenna. (b) Long wavelengths. It is because long wavelengths diffract more than short wavelengths.
78 4. Interference This phenomenon of overlapping 2 waves is called interference. Dippers attached to the same vibrating bar. The dippers act as 2 identical sources called coherent sources The waves produced have the same frequency and wavelength
79 At a point : crest from S 1 crest from S 2 bigger crest constructive interference : trough from S 1 trough from S 2 bigger trough
80 crest from S 1 trough from S 2 cancel out each other, no vibration, no energy destructive interference : Energy redistributes to constructive positions from destructive ones
81 S1S1 S2S2 P Q X Y Analyzing the interference pattern: crest trough At points P,Q : constructive interference occurs At points X,Y : destructive interference occurs antinodal lines: line of C.I. nodal line: line of D.I. Note**: 1.Path difference(e.g. S 1 Q- S 2 Q ) = n, n =0,1,2….., constructive interference occurs. 3.Path difference = (n+1/2), destructive interference occurs
82 P.76 7,8 a.P—CI, Q—CI b.P—CI, Q—DC c.P—CI, Q—DC a.d 1 Q-d 2 Q = 1.5 If final = /2 d 1 Q-d 2 Q = 1.5(2 final )= 3 final C.I. Occurs at Q b.If final = 3 d 1 Q-d 2 Q = 1.5( final /3)= 0.5 final DC occurs at Q
83 P.77 9 (1994), 10 (1997)
84 /(Sloping edges)
85 Note**: 1.Path difference(e.g. S 1 Q- S 2 Q ) = n, n =0,1,2….., constructive interference occurs. 2.Path difference = (n+1/2), destructive interference occurs 3.If f increases ( decreases), nodal lines are more closely spaced. 4.Distance between sources increases, nodal lines are more closely spaced. P. 65
86 P.77 9 (1994), 10 (1997)
’: P.1 of 2
’:P.2 of 2
89 P (2001)
91 P.112 8(1995), 9(1999)
92 P.112 (9)—1999’ a. of TV waves = 0.6 m of radio wave = 500 m b. (i) diffraction (ii) Longer wavelength has greater diffraction. Radio waves have greater diffraction round the hill than that of TV waves because the of radio waves is much longer when compare to TV waves. c. The bad reception is due to the interference of the TV waves reflect from the aeroplane with those coming from the transmitting station. d. (i) path difference = 3.95 – 3.2 = 0.75 km (ii) 750 m = 1.5 of radio wave. Mary cannot listen to the radio broadcasting clearly because destructive interference occurs when waves from stations P and Q arrive at her house. e. To reduce the interference effect between radio waves from different district
93 Laws of reflection: 1.The incident ray, the reflected ray and the normal all lie in the same plane 2.The incident angle = the reflected angle incident ray reflected ray plane mirror angle of incidence angle of reflection normal
94 2 types of reflection: 1.Regular reflection (form clear image) 2.Diffuse reflection (no clear image)
95 Image formed by plane mirror: object plane mirror image
96 N M A’ B’ A B
97 N M A’ B’ A B
98 Properties of images formed by plane mirror: 1.Image distance = object distance 2.Same size as object 3.Erect and laterally inverted 4.virtual P.142 (10)
99 P.142 (10) a.erect, laterally inverted, same size, virtual b.DIY c.= half the height of the boy = 0.75m d.Yes.
100 Refraction of light The bending of light when it travels from one medium (air)to another(water/glass) Note: light bends more towards normal in optically denser medium(larger R.I.). refracted ray incident ray normal angle of incidence angle of refraction air glass
101 Some phenomena: 1.Real and Apparent depth 2.Dispersion 3.Total internal reflection
102 Laws: 1.The incident ray, the refracted ray and the normal all lie in the same plane. 2.(Snell’s Law): Note: i must be in air medium Constant = R.I. = n normal air medium X i r i r
103 Sin i Sin r Slope = R.I.
104 Note: Light is reversible P (1991) P ,2,3,4