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SM Sains Muzaffar Syah
6.2
  Occurs when a wave strikes an obstacle. What is Reflection of wave?  The wave undergoes a change in direction of propagation when it is reflected.  The value of frequency (f), wavelength (λ) and speed (v) remain the same after reflection. Incident wave : the wave before it strikes the obstacle State the Law of Reflection: Reflected wave: the wave which has undergone a change in direction of propagation after reflection. The angle of incidence, i is equal to the angle of reflection, r. i = angle of incident – the angle between the direction of propagation of incident wave and the normal r = angle of reflection – the angle between the direction of propagation of reflected wave and the normal.

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

 Aim: To observe the reflection of plane water waves in a ripple tank Apparatus: 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 set the vibrating. Increase the frequency of the waves by increasing the voltage power supply to the motor. 3. Observe the reflected wave by using a stroboscope. Draw a diagram to show reflection of waves.

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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 the wavelength of the incident water waves? 2. When the speed of the motor increases, what will happen to the separation between adjacent bright lines or dark lines?

Activity 2: Reflection of sound waves

Aim

Apparatus

Procedure

To investigate the reflection of sound waves

2 cardboard tubes, stopwatch, a softboard, a wooden board with a smooth surface and a protactor.

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

The tube is place at an angle to a reflecting surface.

Keep a softboard vertical on the table top.

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

Adjust the angle of the second tube until a loud ticking is heard.

Discussion

Conclusion

i
(°)
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?

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

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6.3

What is

Refraction of

waves?

What happens

to frequency,

speed,

wavelength &

direction?

Use the

words,

‘increase’,

decrease’ or

‘unchanged’

How does the direction of waves change when:

How to draw a diagram to show the refraction of waves?

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

f is constant v is directly proportional to λ

 tan 1 v 2  cons t  

2

v

f

v v

 

1

The relationship between v and λ of a water wave in deep and shallow water:

v = fλ

Water waves passes from shallow water to deep water

Characteristics

Speed

Wavelength

Frequency

Water passing from the shallow region to the deep region, the water wave is refracted away from the normal.

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

Characteristics

Speed

Wavelength

Frequency

Water waves passes from deep water to shallow water

Water passing from the deep region to the shallow region, the water wave is refracted toward the normal.

 1 2 3 4 Draw the refracted wavefronts.

Draw a line to represent the direction of propagation of the incident waves.

Draw the normal, N at the boundary between the two areas.

Draw the line to represent the direction of propagation of the refracted waves

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Activity 2: Refraction of Water Waves

 Use a ripple tank Draw a ray diagram to show refraction of waves. Discussion 5. 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 the wave change? (i) Direction: (ii) Wavelength: (iii) Speed: Conclusion Explain the refraction of water.

Exercise 1

1.

(a)

(b)

A plane wave has a wavelength of 2 cm

and a velocity of 8 cm s

the surface of shallow water. When the

plane wave moves into an area of greater

depth, its velocity becomes 12 cm s What is

-1

as it moves over

-1

.

the wavelength

the frequency of the wave in the area of greater depth?

2. The diagram shows a plane water wave moving from one area P to another area Q of different 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 shape of the beach.

It is because the water waves propagate from

 the water to the water. The speed and wavelength as the

waves approach the beach and hence the water

wave is refracted

the normal.

The water in the bay stationary compared to the

water at the cape. It is because the depth of

water varies slowly across the area of the bay

and the

cape. The amplitude of the water wave near the bay is low and hence the water at the bay is comparatively still.

of the water wave spreads to a wider area compared to the region near the

Refraction of Light Waves

1. When a ray propagates from an

optically less dense medium to an optically denser medium, the ray

refracts

the normal.

2. The speed of light

as it

propagates in the glass block, causing it to change the direction of propagation.

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. the cooler layer of air 4 On a cool night, the sound waves travel (slower/faster)in

near the surface of the Earth than in the upper, warmer air. The waves are refracted

towards 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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6.4

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 more circular

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 pattern of the waves.

3. The energy decrease.

of propagation and the

of the diffraction wave decreases so its

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 enough

2. the wavelength is large enough.

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Activity 5: Diffraction of water waves in a ripple tank

Aim

Apparatus

Procedure

To study the characteristics of the diffracted waves.

Ripple tank, lamp, power pack, stroboscope

A ripple tank is filled with water and set up as shown.

Switch on the power pack.

Use a barrier to block the incident straight water waves. Observe the wave pattern beyond the barrier.

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

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

Observation
(a) Wide gap
(b) Narrow gap
The waves are …………. only at the edges
after passing through the gap.
The effect of diffraction is ……………………
The waves are …………….and appear to
originated from the small gap.
The effect of diffraction is …………
Straight water wave propagate towards an
obstacle.

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Questions

1. 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 and are 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 the gap?

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 adjacent rooms.

We can hear the sound of a radio placed nearby a

corner of a wall but we cannot see the radio. Why?

Sound waves are more easily diffracted in comparison to light waves because the wavelength

of sound waves is much wavelength of light waves.

than the

Diffraction of light Light 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 the wavelengths of light are very short.

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OBJECTIVE QUESTIONS

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

2 Which of the following characteristics of

waves, changes when the waves are reflected?

(2007)

A. Direction of propagation

B. Wavelength

C. Frequency

D. Speed

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

 Speed Wavelength Frequency A increases Increases Decreases B Constant Decreases Increases C Decreases Decreases Constant D Increases Increases constant

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

A. speed

B. wavelength

C. frequency

D. intensity

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

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

A. The water density in region P is higher than in region Q.

B. Region P is deeper than region Q

C. Speed of water in region Q is larger than region P

D. Amplitude of the water waves in region Q is higher than region P

6 The diagram shows a refraction of a water wave.

If the speed of the water wave is 5 cms -1 in the region of deep water, calculate the speed of the water 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 ripple tank containing a uniform depth of water. The wavelength of a water wave changes when the frequency of the wave changes. Which of the following graphs shows the correct relationship between the wavelength and frequency of the wave?

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8 A ripple tank is not leveled so the depth of water in the tank is not uniform. Water at X is deeper than Y.

If the straight dipper vibrates to produce plane waves, which diagram shows the correct path of the wave when it reaches P?

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

What happens to the water waves after passing the island?

 Velocity Wavelength amplitude A Increases Decrease No change B Decreases Increases Increases C No change No change Decreases D No change No change No change

10 Which of the diagram below shows the of water waves which passes through a small gap correctly?

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The diagram shows the pattern of water approaching a shoreline. Which is the most suitable locations for a child to swim?

12.

Diagram 3 shows a ripple tank with a sloping base and a vibrator being vibrated.

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

13.

A tilted basin contains water. Water is dripped at a constant rate into the basin as shown in the diagram below.

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

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

Which statement is correct about the reflected and incident waves? (2005)

A. The speed of the reflected waves is the same as the speed of the incident waves

B. The wavelength of the reflected waves is shorter than that of the incident waves

C. The frequency of the reflected waves is lower than that of the incident waves

D. The directions of the reflected waves are always at right angles to the incident waves.

STRUCTURE QUESTIONS

1. Figure 1 shows a water waves being

generated by a plane dipper oscillating in a ripple tank with a deep area and a shallow

area.

Figure 1

 (a) State two changes when the wave enters the shallow area. (b) If the frequency of the oscillating plane dipper is 5 Hz, the wavelength of the waves in the deep area is 4.0 cm and the wavelength of the waves is the shallow area is 3.0 cm, calculate:

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

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

 (ii) the speed of the waves in the deep area. (iii) The speed of the waves in the shallow area.

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

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(a) By drawing a straight line on the figure above mark the boundary of the perspex plat.

(b) (i)Determine the wavelength of the waves before travels over the perspex plat.

(ii)

(iii)

Calculate the frequency of the waves

before travels over the perspex plat

when the velocity is 40 cms

-1

.

Determine the velocity of the waves travels over the perspex plat.

3. Figure 1 shows a water waves in a ripple tank travels over a perspex plat. Figure 2 shows a water waves propagated from 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 water between A and B. (iii) name a physical quantity is unchanged .

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

(c) Name the phenomenon in (b)

(d) The water waves with a wavelength of

1.5 m travels with velocity 2.0 ms from the sea to the beach. The velocity of the waves when reach near the

beach is 1.2 ms -1 .Calculate

-1

(i) the frequency of the waves

(ii) the wavelength of the waves near the beach

4. Figure 3 shows a tank with three areas of water with different depths. A dipper produces water waves in region Z and is moving to region Y and X. Draw a diagram to show how the waves propagate through the three different area.

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

 (a) Draw the pattern of the waves after passing through the opening between the walls. (b) Name the wave phenomena shown in the diagram. (c) Compare the size of the water wave before and after passing through the gap? (d) By relating the size of the water wave with the energy of the wave, explain how does the wall protect the port from being hit by large waves.

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 the wave patterns and the wavelength of the waves before and after they pass through the gaps. Relate the size of the gaps, the wave patterns and the wavelength to deduce a relevant physics concept.

(iii)Name the wave phenomena shown in Figure 6.1.

7. The figure below shows water waves propagated 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. 6. Figures 6.1 and 6.2 shows plane waves produced by linear dipper oscillating in a ripple tank passing through gaps formed by two straight barriers. Figure 6.1 shows waves passing through a wide gap and Figure 6.2 shows waves passing through a (c) With the use of apparatus such as ripple tank and other apparatus, describe an experimental framework to test your hypothesis. In your description, state clearly the following: narrow gap. (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 of controlling the manipulated variable and the method of (vi) measuring the responding variable Way you would tabulate the data Figure 6.1 (vii) Way you would analysis the data

(SPM 2003)

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