1 wave motion ( 波動 ) wave motion is a kind of vibration(oscillation), through which, energy (not...

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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.

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Vibration of water molecules perpendicular to wave(energy) travelling direction.

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Why can ocean waves lift a ship?

Waves carry energy

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Tsunamis carry a huge amount of energy

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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

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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

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vibration of particleslight

Some e.g.

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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.

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2. Longitudinal wave: vibration is parallel to wave traveling direction.

rarefactionCompression

Simulation of longitudinal wave: CH 10

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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 > 20000 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

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To prove that sound is a longitudinal wave.

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1. Amplitude: maximum displacement of particles from equilibrium

position.

2. The bigger the amplitude, the greater the energy transferred

Simulation: CH 10

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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

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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

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2006

V = f λ=(2)(100) = 200 ms-1

(= 720 km h-1)

V = distance/ time

t = 1500/720 = 2 hours

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Phase ( 相位 ) 。

In phase ( 同相 ) : vibrations that are in steps

anti- 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

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at rest37.5 cm12.5 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.

(37.5 + 12.5)÷0.5 = 100 cm/s

f = V ÷ λ = 100 ÷ 15 = 6.7 Hz

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地震可按照震源深度分為淺源地震、中源地震和深源地震。淺源地震大多發生在地表以下 30 公里深度以上的範圍內，而深源地震最深的可以達到 650 公里左右。其中，淺源地震的發震頻率高，佔地震總數的 70% 以上

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If M Richter magnitude scale ( 黎克特制 )is increased by 1, E ( 能量 )is magnified by a factor of 101.5 (=~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 = 34359738368 ~ 30000000000 ~ 300 億倍能量Earth's daily receipt of solar energy

~ M = 12

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Richter Effect

Less than 3.5 not felt, but recorded.

3.5-5.4 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.

6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live. 7.0-7.9 Major earthquake. Can cause serious

damage over larger areas.

8 or greater Great earthquake. Cause serious damage in areas several

hundred kilometers across.

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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 rec

orded

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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

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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

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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.

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Displacement from equilibrium position

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四川汶川發生黎克特制 7.8 級地震，威力相當於二百五十六顆原子彈，震動大半個中國，遠至港澳台、泰國、越南都有震感。死亡人數已增加至萬人

中國地震局地震預測研究所研究員張國民稱，汶川發生地震是屬於淺源地震 ( 約 10 公里 ) ，破壞力度較大。

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Describing waves with graphs:

1. Displacement – distance graph of a wave

2. Displacement – time graph of a wave

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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

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Use displacement – distance graph to determine the motion of particles

displacement / cm

distance / cm

1

–1

02 4 6 8

PQ

RS

direction of travel

later

present

P : moving downwards

R : moving upwards

Q : momentarily at rest.

S : momentarily at rest.

Traveling direction must be given

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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-

1. +

-

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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.

disp

lace

men

t

0 -0 .7 -1 .0 -0 .7 0 0 .7 1 .0 0 .7 0 - 0 .7 e tc

p ro p a g a tio n a t 8 c m s

p o s it io n

-1

The displacement - time graph for particle 13 is shown below.

1 period

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Traveling longitudinal wave (P.23):

Traveling direction must be given

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Displacement-distance graph of longitudinal waves(P.27):The Displacement-distance graph of longitudinal waves looks like a transverse wave!

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Class work:

P.33

Questions: 1-8

HW:

P.34

7,8,9,10,11

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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/sT

amplitude

period

2T

1

0

–1

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P.27

4.

P.28

4,5,6,7

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P.35

1990

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P.35

1998

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P.35

2002

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E

F

G

H

I J K L MA B C D N O

5 cm

E F G H I J K L MA B C D

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.

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displacement / cm

distance

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6

A G MD

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

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4

0

4

distance / cm

displacement / cm

travelling direction

A

B C

D

5 10 15 20

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.

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4

0

displacement / cm

A

0.2

0.4

4

time / s

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e.g.

P26 check point

1 – 4

HW

P28

4,5,6,7

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http://eant01.hkeaa.edu.hk/hkea/switch.asp?p_left=hkcee_left.asp&p_clickurl=exam_syllabuses.asp?p_exam=HKCEE

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Properties of Waves :

1. Reflection

2. Refraction

3. Diffraction

4. Interference

Use water waves to demonstrate wave properties.

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Water waves work as lens .

Ripple tank

crest:converge light rays

water

Trough : diverge light rays

Ripple tank ( 水波槽原理 )

brightdark

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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

Wavefront Rays

Rays

Rays

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Use water waves to demonstrate 4 properties of wave ：1.reflection

2.refraction

3.diffraction

4.interferenceStraight wave

Circular wave

1. reflection:

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Law of reflection:

= incident angle reflection angle

i = r

Incident ray

Normal

ir

Reflected ray

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P.55 check point

1,2, 3,

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Phase change in refraction

P.57 check point

1-3

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2. refraction:

When wave travels from one medium to another, its speed will change, this leads to refraction.

Straight wave 1

Straight wave 2

shallower

deeper

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Travelling direction

1 2 1 > 2

decreases

f unchanged

v decreases

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Shallower:

1. decreases

2. V decreases

3. rays bend towards normal

normal

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They converge to a point (focus).

shallow convex region:

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They diverge from a point (focus).

shallow concave region:

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P.55

1,2,3,4

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P 75 (5)

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3. diffraction

Diffraction is the bending of waves around obstacles.

Diffraction around an edge:

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increases, degree of diffraction increases.

Degree of diffraction

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Diffraction through a slitThe degree of diffraction increases if:

2. the slit width decreases1. increases

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e.g. of diffraction:

1. Sound waves bend around the rim of loudspeakers

2. TV broadcasting: TV antennas must point directly to transmitting station.

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P.76

6

1. Decrease the depth of water

2. Increase the width of the slit

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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)

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(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.

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4. InterferenceThis 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

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At a point :

crest from S1 crest from S2 bigger crest

constructive interference :

trough from S1 trough from S2

bigger trough

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crest from S1

trough from S2

cancel out each other,no vibration, no energy

destructive interference :

Energy redistributes to constructive positions from destructive ones

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S1 S2

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.S1Q- S2Q ) = n , n =0,1,2….. ,

constructive interference occurs.

3. Path difference = (n+1/2) , destructive interference occurs

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P.76

7,8a. P—CI, Q—CI

b. P—CI, Q—DC

c. P—CI, Q—DC

a. d1Q-d2Q = 1.5

If final = /2

d1Q-d2Q = 1.5(2 final)= 3 final

C.I. Occurs at Q

b. If final = 3

d1Q-d2Q = 1.5(final/3)= 0.5 final

DC occurs at Q

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P.77

9 (1994),

10 (1997)

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/(Sloping edges)

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Note**:1. Path difference(e.g.S1Q- S2Q ) = 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

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P.77

9 (1994),

10 (1997)

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1997’: P.1 of 2

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1997’:P.2 of 2

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P.78--11 (2001)

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2001

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P.112

8(1995),

9(1999)

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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

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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

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2 types of reflection:

1. Regular reflection (form clear image)

2. Diffuse reflection (no clear image)

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Image formed by plane mirror:

object plane mirror image

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N

M A’

B’

A

B

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N

M A’

B’

A

B

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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)---1995

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P.142 (10)---1995

a. erect, laterally inverted, same size, virtual

b. DIY

c. = half the height of the boy = 0.75m

d. Yes.

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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

airglass

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Some phenomena:1. Real and Apparent depth

2. Dispersion

3. Total internal reflection

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Laws:

1. The incident ray, the refracted ray and the normal all lie in the same plane.

2. (Snell’s Law):

constantsin

sin

X

a

Note:

• i must be in air medium

• Constant = R.I. = n

normal

air

medium X

i

r

i

r

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Sin i

Sin rSlope = R.I.

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Note: Light is reversible

P.175 ----9(1991)

P.174----- 1,2,3,4

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Total internal reflection.

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119

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