Waves

SM Sains Muzaffar Syah

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What is Reflectionof wave?

Occurs when a wave strikes an obstacle.

The wave undergoes a change in direction of propagation when it isreflected.

The value of frequency (f), wavelength (λ) and speed (v) remain the same after reflection.

Incident wave : the wave before it strikes the obstacle

Reflected wave: the wave which has undergone a change indirection of propagation after reflection.

i = angle of incident – the angle between the direction ofpropagation of incident wave and the normal

r = angle of reflection – the angle between the direction ofpropagation of reflected wave and the normal.

State the Law of Reflection:

The angle of incidence, i is equalto the angle of reflection, r.

Activity 1: Reflection of plane water waves in a ripple tank

Aim: To observe the reflection of plane water waves in a ripple tankApparatus: Ripple tank, plane reflector, white paper, wooden bar, lamp, motor, power supply,

sponge and mechanical stroboscope.Procedure 1. Set up a ripple tank.

2. Switch on the motor to setthe vibrating. Increase thefrequency of the waves byincreasing the voltagepower supply to the motor.

3. Observe the reflected waveby using a stroboscope.

Draw adiagram toshowreflection ofwaves.

6.2


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Conclusion What will be the direction of travel of the reflected waves when:(a) the incident water waves travel perpendicular to the barrier?

(b) the incident water waves travel at an angle to the barrier?

Questions 1. What is the wavelength of the reflected water waves compare to thewavelength of the incident water waves?

2. When the speed of the motor increases, what will happen to the separationbetween adjacent bright lines or dark lines?

Activity 2: Reflection of sound waves

Aim To investigate the reflection of sound wavesApparatus 2 cardboard tubes, stopwatch, a softboard, a wooden board with a smooth

surface and a protactor.Procedure

Place a stop watch at the end of a hollow tube.

The tube is place at an angle to a reflectingsurface.

Keep a softboard vertical on the table top.

Place another hollow tube on the other side ofthe softboard and hold it close to your ear.

Adjust the angle of the second tube until a loudticking is heard.

Discussioni (°) 20 30 40 50 60

r (°)

What is the purpose of the soft board?

How is the angle of incidence related to the angle of reflection?

Conclusion In the reflection of sound, the angle of incidence is ……………… to the angle ofreflection.


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What isRefraction ofwaves?

Refraction of waves is a change of direction in the propagation of waves from onemedium to another due to a change of speed.

The frequency of the waves is equal to the frequency of the source of vibration andhence it remains unchanged. But the speed, wavelength and direction of propagationchange.

What happensto frequency,speed,wavelength &direction?

The relationship between v and λ of a water wave in deep and shallow water:v = fλ f is constant

v v is directly proportional to λ

2

2

1

1tan

vvtcons

vf

Use thewords,‘increase’,decrease’ or‘unchanged’

CharacteristicsWater wavespasses fromdeep water toshallow water

CharacteristicsWater wavespasses from

shallow water todeep water

Speed Speed

Wavelength Wavelength

Frequency Frequency

How does thedirection ofwaves changewhen:

Water passing from the deep region tothe shallow region, the water wave isrefracted toward the normal.

Water passing from the shallow region to thedeep region, the water wave is refractedaway from the normal.

How to drawa diagram to

show therefraction of

waves?

1. Draw a line to represent the direction of propagation of theincident waves.

2. Draw the normal, N at the boundary between the two areas.3. Draw the line to represent the direction of propagation of the

refracted waves4. Draw the refracted wavefronts.

6.3


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Activity 2: Refraction of Water Waves

Use a rippletank

Draw a raydiagram toshowrefraction ofwaves.

Discussion5. Compare the wavelength for the water wave in the deep region and the shallow region.

6. Compare the frequency of the water wave in the deep region and the shallow region.


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7. By using the answer in question 1 & 2, and formula v = fλ, compare the speed of the water waves in the two regions.

8. How does the direction of waves change when passing from the deep region to the shallow region?

9. When waves pass from the deep region to the shallow region, how do the following properties of thewave change?

(i) Direction:

(ii) Wavelength:

(iii) Speed:

ConclusionExplain the refraction of water.

Exercise 1

1. A plane wave has a wavelength of 2 cmand a velocity of 8 cm s

-1as it moves over

the surface of shallow water. When theplane wave moves into an area of greaterdepth, its velocity becomes 12 cm s

-1.

What is

(a) the wavelength(b) the frequency of the wave in the area of

greater depth?

2. The diagram shows a plane water wavemoving from one area P to another area Q ofdifferent depth.

If the speed of water wave in P is 18 cm s-1

,what is the speed of water wave in Q?


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Refraction of water waves of sea water When the water waves propagated from the sea

to the beach , the water waves follow the shapeof the beach.

It is because the water waves propagate fromthe _______water to the ___________ water. Thespeed and wavelength ___________ as thewaves approach the beach and hence the waterwave is refracted __________ the normal.

The water in the bay stationary compared to thewater at the cape. It is because the depth ofwater varies slowly across the area of the bayand the _______ of the water wave spreads to a wider area compared to the region near thecape. The amplitude of the water wave near the bay is low and hence the water at the bay iscomparatively still.

Refraction of Light Waves

1. When a ray propagates from anoptically less dense medium to anoptically denser medium, the rayrefracts ___________ the normal.

2. The speed of light ____________ as itpropagates in the glass block, causingit to change the direction ofpropagation.

Refraction of Sound Waves

Figure (a) Figure (b)

1. Sound waves travel faster in warm air than in _______ (cold/hot) air.2. On a hot day, the hot surface of the Earth causes the layer of air near the surface to be

__________.(warmer/cooler)3. This causes sound waves to be ____________ away from the Earth.4. On a cool night, the sound waves travel _________(slower/faster)in the cooler layer of air

near the surface of the Earth than in the upper, warmer air. The waves are refractedtowards the Earth. Hence, sound can be heard over a longer distance on a cold_________(night/day) compared with a hot day.


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What is diffraction of waves?Diffraction of waves is a phenomenon in which waves spread out as they pass through a ______or round a small ____________.

The waves bend and change direction near the edges of the gap.

Small gap – more obvious large gap – less obvious

The effect of diffraction is obvious if the shape of the diffracted waves more spread out or morecircular

Small barrier – more obvious Large barrier – less obvious

What are Characteristics of diffracted waves?1. Frequency, wavelength and speed of waves do not

change.2. Changes in the ___________ of propagation and the

pattern of the waves.3. The ___________ of the diffraction wave decreases so its

energy decrease.

What are the factors that influence the effect of diffraction?The effect of diffraction is obvious if:1. the size of the gap or obstacle is small enough2. the wavelength is large enough.

6.4


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Activity 5: Diffraction of water waves in a ripple tank

Aim To study the characteristics of the diffracted waves.Apparatus Ripple tank, lamp, power pack, stroboscopeProcedure

A ripple tank is filled with water and set up asshown.

Switch on the power pack.

Use a barrier to block the incident straightwater waves. Observe the wave patternbeyond the barrier.

Send a straight water waves to pass through agap. Observe the pattern of diffracted wavesbeyond the gap.

Send straight water waves towards a smallgap. Observe the wave pattern beyond thesmall gap.

Observation(a) Wide gap

The waves are …………. only at the edgesafter passing through the gap.The effect of diffraction is ……………………

(b) Narrow gap

The waves are …………….and appear tooriginated from the small gap.The effect of diffraction is …………..

Straight water wave propagate towards anobstacle.


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Questions1. Is there any difference in the wavelengths of the incoming waves and the diffracted

waves?

2. What can you deduce about the frequencies if the waves come from the same source andare propagated in equal depths of water?

3. Will diffraction affect the amplitude of the diffracted waves?

4. What is the relationship between the shape of the diffracted waves with the size of thegap?

Conclusions

As the size of the gap or obstacle …………………, the effect of diffraction becomes obvious.

Diffraction of sound Sound diffracting around corners so allowing us to

hear others who are speaking to us from adjacentrooms.

We can hear the sound of a radio placed nearby acorner of a wall but we cannot see the radio. Why?

Sound waves are more easily diffracted incomparison to light waves because the wavelengthof sound waves is much __________ than thewavelength of light waves.

Diffraction of lightLight is diffracted if it passes through a narrow slit comparable in size to its wavelength.However, the effect is not obvious as the size of the slit increases. This is because thewavelengths of light are very short.


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OBJECTIVE QUESTIONS

1 Which diagram shows the correct pattern ofreflected water waves? (2004)

2 Which of the following characteristics ofwaves, changes when the waves are reflected?(2007)A. Direction of propagationB. WavelengthC. FrequencyD. Speed

3 Which of the following shows the effect of awave when a water wave is moving from a deepregion to a shallow region?

Speed Wavelength FrequencyA increases Increases DecreasesB Constant Decreases IncreasesC Decreases Decreases ConstantD Increases Increases constant

4 When a light ray from air enters a glass, thelight ray is refracted into the glass. Which ofthe following quantities remains constant whenthe light ray is refracted into the glass?

A. speedB. wavelengthC. frequencyD. intensity

5 The diagram shows a water waves propagatefrom region P to region Q in a ripple tank.

Which statements explains the changing of thedirection of the water waves?

A. The water density in region P is higherthan in region Q.

B. Region P is deeper than region QC. Speed of water in region Q is larger

than region PD. Amplitude of the water waves in region

Q is higher than region P

6 The diagram shows a refraction of a waterwave.

If the speed of the water wave is 5 cms-1

in theregion of deep water, calculate the speed of thewater wave in the region of shallow water.

A. 1.5 cms-1

B. 2.5 cms-1

C. 3.0 cms-1

D. 5.0 cms-1

E. 5.2 cms-1

7 A series of plane waves is produced in a rippletank containing a uniform depth of water. Thewavelength of a water wave changes when thefrequency of the wave changes. Which of thefollowing graphs shows the correctrelationship between the wavelength andfrequency of the wave?


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8 A ripple tank is not leveled so the depth ofwater in the tank is not uniform. Water at X isdeeper than Y.

If the straight dipper vibrates to produce planewaves, which diagram shows the correct pathof the wave when it reaches P?

9 The diagram shows water waves approachingan island in the South China Sea.

What happens to the water waves after passingthe island?

Velocity Wavelength amplitudeA Increases Decrease No changeB Decreases Increases IncreasesC No change No change DecreasesD No change No change No change

10 Which of the diagram below shows the of waterwaves which passes through a small gapcorrectly?

11The diagram shows the pattern of waterapproaching a shoreline. Which is the mostsuitable locations for a child to swim?

12. Diagram 3 shows a ripple tank with a slopingbase and a vibrator being vibrated.

Which of the following wave patterns can beseen on the screen? (2006)

13. A tilted basin contains water. Water isdripped at a constant rate into the basin asshown in the diagram below.

Which pattern of the wavefronts will beobserved in the basin? (2005)


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14. The diagram shows the wavefronts of aplane wave incident on a plane reflector.

Which statement is correct about thereflected and incident waves? (2005)A. The speed of the reflected waves is the

same as the speed of the incident wavesB. The wavelength of the reflected waves is

shorter than that of the incident wavesC. The frequency of the reflected waves is

lower than that of the incident wavesD. The directions of the reflected waves are

always at right angles to the incidentwaves.

15. Diagram below shows water wavespropagating in an area of different depths.

Which of the following diagrams shows thepropagation of the waves correctly? (2007)

STRUCTURE QUESTIONS

1. Figure 1 shows a water waves beinggenerated by a plane dipper oscillating in aripple tank with a deep area and a shallowarea.

Figure 1

(a) State two changes when the wave enters theshallow area.

(b) If the frequency of the oscillating planedipper is 5 Hz, the wavelength of the wavesin the deep area is 4.0 cm and thewavelength of the waves is the shallow areais 3.0 cm, calculate:

(ii) the speed of the waves in the deeparea.

(iii) The speed of the waves in the shallowarea.

2. The figure shows successive wavefronts ofwaves traveling on the surface of water aripple tank.A perspex plat is placed on the bottom of aripple tank .


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(a) By drawing a straight line on the figureabove mark the boundary of theperspex plat.

(b) (i)Determine the wavelength of thewaves before travels over theperspex plat.

(ii) Calculate the frequency of the wavesbefore travels over the perspex platwhen the velocity is 40 cms

-1.

(iii) Determine the velocity of the wavestravels over the perspex plat.

3. Figure 1 shows a water waves in a rippletank travels over a perspex plat.Figure 2 shows a water waves propagatedfrom the sea to the beach.

Figure 1 Figure 2

(a) Based on Figure 1 and Figure 2(i) state the change in wavelength of

the water waves from A to B.

___________________________

___________________________

(ii) compare the depth of waterbetween A and B.

___________________________

(iii) name a physical quantity isunchanged .

___________________________

(b) Relate your answer in (a)(i) and (a)(ii) todeduce a wave phenomenon.

(c) Name the phenomenon in (b)

________________________________

(d) The water waves with a wavelength of1.5 m travels with velocity 2.0 ms

-1

from the sea to the beach. The velocityof the waves when reach near thebeach is 1.2 ms

-1.Calculate

(i) the frequency of the waves

(ii) the wavelength of the waves nearthe beach

4. Figure 3 shows a tank with three areas ofwater with different depths. A dipperproduces water waves in region Z and ismoving to region Y and X. Draw a diagram toshow how the waves propagate through thethree different area.


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5. The diagram shows water waves passingthrough the entrance of a model harbour.

(a) Draw the pattern of the waves afterpassing through the opening betweenthe walls.

(b) Name the wave phenomena shown inthe diagram.

______________________________

(c) Compare the size of the water wavebefore and after passing through thegap?

______________________________

______________________________

(d) By relating the size of the water wavewith the energy of the wave, explainhow does the wall protect the port frombeing hit by large waves.

______________________________

______________________________

______________________________

6. Figures 6.1 and 6.2 shows plane wavesproduced by linear dipper oscillating in aripple tank passing through gaps formed bytwo straight barriers. Figure 6.1 showswaves passing through a wide gap andFigure 6.2 shows waves passing through anarrow gap.

Figure 6.1

Figure 6.2

(a) (i) State a formula relating the speed, v,wavelength, λ and frequency of a wave, f.

(ii) Using Figures 6.1 and 6.2, compare thewave patterns and the wavelength of thewaves before and after they pass throughthe gaps. Relate the size of the gaps, thewave patterns and the wavelength todeduce a relevant physics concept.

(iii)Name the wave phenomena shown inFigure 6.1.

7. The figure below shows water wavespropagated from the sea towards the shore.

Observed the wave pattern and the sea-bed.Based on the observations:(a) State one suitable inference that can be

made.(b) State one appropriate hypothesis for an

investigation.(c) With the use of apparatus such as

ripple tank and other apparatus,describe an experimental framework totest your hypothesis.In your description, state clearly thefollowing:(i) Aim of the experiment(ii) Variables in the experiment(iii) List of apparatus and materials(iv) Arrangement of the apparatus(v) The procedure of the experiment

which include the method ofcontrolling the manipulatedvariable and the method ofmeasuring the responding variable

(vi) Way you would tabulate the data(vii) Way you would analysis the data

(SPM 2003)

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Waves