New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
CHAPTER 6 WAVE PHENOMENA
1
Introduction
Four wave phenomena
Representing water wave
[Concept]
In a ripple tank experiment, a set of waves is produced.
(a) Sketch the wave pattern if the vibrating source is a straight bar.
straight
bar
(b) Sketch the wave pattern if the vibrating source is a dipper.
dipper
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
2
Reflection
[Concept]
Draw the wave pattern after it is reflected in the following situations:
(a)
(b)
[Further question]
[HKCEE]
During experiment, how to prevent water waves from bouncing back at the edges of the tank?
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
3
Refraction
Wave equation
[Concept]
A dipper is generating water waves in a tank. The generated wave follows the wave equation:
v = fλ
What is the experimental parameter that controls f:
What is the experimental parameter that controls v:
How about λ?
Refraction in 1D
[Concept]
A train of water wave in region A enters region B on normal incident. It is known that region B is shallower
than region A. Draw the wave pattern.
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
Refraction in 2D
[Concept]
A train of water wave in region A enters region B on oblique incident. It is known that region B is
shallower than region A. Draw the wave pattern.
[Refraction][HKCEE]
A series of plane waves travel from medium A into
medium B. The figure above shows the positions, PQ
and P'Q', of a wave front before and after entering the
medium B. What would happen to the speed and the
wavelength of the wave when it travels from medium A
to medium B?
A
B
C
D
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Speed
Increases
Increases
Decreases
Decreases
Wavelength
Increases
Remains unchanged
Decreases
Remains unchanged
New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
[Refraction drawing]
A vibrator generates continuous plane in a ripple tank (Figure (a)). Figure (b) shows the wave pattern
observed. Now the ripple tank is tilted as shown in Figure (c).
Draw the wave pattern will be observed.
[Refraction calculation]
Consider the following case. The wavelength reduces from 3 m to 2 m when the wave travels from region A
to region B.
3m
2m
Region B
Region A
–1
If the wave speed in the faster region is found to be 1.5 m s , find the wave speed in the slower region.
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
4
Diffraction
[Concept]
Draw the diffraction pattern of the water wave for the following cases:
(a)
(b)
Degree of diffraction
The degree of diffraction depends positively on the parameter: ____________________.
As a comparison:
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
5
Interference
Interference in terms of phase
[Concept]
For two waves arrive in phase,
For two waves arrive in antiphase,
Interference in water tank
[Concept]
In the figure, solid lines represent the crests. Locate the positions of constructive interference by ∆ , and
positions of destructive interference by × .
Hence draw the nodal lines and antinodal lines.
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
Interference in terms of path difference
[Concept]
For constructive interference:
For destructive interference:
[Interference][HKCEE]
A dipper S1 is connected to a vibrator and produces circular
water waves in a ripple tank. A cork is placed at a point P on
the water surface as shown in Figure 3.
(a) Describe the motion of the cork as the water waves pass
through it. (1 mark)
(b) Suppose another dipper S2 is connected to the same vibrator and produces identical water waves. It is
known that S1P = 6.0 cm, S2P = 7.8 cm and the wavelength of the water waves is 1.2 cm.
(i)
Name the wave phenomenon that occurs when both dippers vibrate. (1 mark)
(ii)
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How would the motion of the cork be affected? Explain your answer. (3 marks)
New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
[Interference][HKCEE]
Two dippers S1 and S2 vibrate in phase producing identical circular water waves in a ripple tank. Figure 3
shows the wave pattern at a certain instant. (Note: The dark lines represent crests.) The distance between S1
and S2 is 0.06 m and it is known that the water waves travel with a speed of 0.4 ms–1.
(a) Find the wavelength and frequency of the water waves. (3 marks)
(b) The ripple tank has a spongy lining at its edges. Explain the function of the spongy lining. (2 marks)
(c) P and Q are two points at the water surface as shown in Figure 3. Find the path difference at
(i) point P, and (ii) point Q
from S1 and S2, giving the answers in terms of the wavelength λ of the water waves. Hence state the types of
interference occurring at P and Q. (4 marks)
*(d) How would the interference at Q be affected if the frequency of vibration of the two dippers is
doubled? Explain your answer. (Note: You may assume that the speed of the water waves remains
unchanged.) (4 marks)
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
6
Stationary waves
Reflection and phase change
[Concept]
Draw the reflected wave in the following cases:
Fixed end:
Free end:
Frequencies for stationary waves
[Concept]
A string of length 1m is connected to a vibrator and a fixed end. Determine the first three frequencies that
can generate stationary waves on the string. It is known that the wave travel along the string at a speed of
10m/s.
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
[Stationary wave]
A stationary wave is produced on an elastic string which is fixed with one end to a vibrator and the other to
a stand. The frequency of the vibrator is set at 50 Hz. The distance between the vibrator and the stand is
1m. The figure below shows the positions of particles X and Y at the instant when they are furthest from
their equilibrium positions.
1m
Y
direction of
vibration
X
vibrator
(a) Find the wavelength and the wave speed of the stationary wave.
(3 marks)
(b) In the figure, label an antinode with A and a node with N.
(2 marks)
(c) Describe the motion of particles X and Y at the instant shown.
(2 marks)
(d) The figure below shows the displacement−time graph of particle X.
displacement
time
Sketch the displacement−time graph of particle Y in the same figure. (2 marks)
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New Senior Secondary Physics Compulsory: Wave Motion
Chapter 6 Wave Phenomena
FINAL REMARKS
This chapter introduces four phenomena for waves, namely reflection, refraction, diffraction and interference.
Students should note that diffraction and interference are phenomena that specific to waves, while reflection and
refraction also happen in particles. This is very important in concluding light is a wave.
The most important topic in this chapter is interference. The determination of which kind of interference will
happen using path difference is a topic very usually examined. Students should be familiar with the calculation
involved.
For stationary wave, student should note that this is not a new kind of wave, but rather a result of superposition of
two oppositely travelling waves.
APPENDIX 1: STABLE INTERFERENCE PATTERN
[Concept]
How to generate a stable interference pattern?
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