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Chapter1: Waves1.4 Analysing Diffraction ofWaves
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1.4 Analysing Diffraction of Waves
1 Diffraction of waves is a phenomenon in whichwaves spread outas they pass through an apertureor
round a small obstacle.
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1.4 Analysing Diffraction of Waves
2 The effect of diffraction is obvious only if
(a) the size of the aperture or obstacle is smallenough,
(b) thewavelengthis large enough.
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1.4 Analysing Diffraction of Waves
3 Characteristics of diffracted waves:
(a) Frequency, wavelength and speed of wavesdo not change.
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1.4 Analysing Diffraction of Waves
3 Characteristics of diffractedwaves:
(b) Changes in the directionof propagationand thepattern of the waves.
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1.4 Analysing Diffraction of Waves
Experiment 1.6: to investigate the pattern ofdiffracted water waves
(I) Fixed wavelength
Problem statement
What is the relationship between the size of the
aperture or obstacle and the effects ofdiffraction of water waves?
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1.4 Analysing Diffraction of Waves
(I) Fixed wavelength
Hypothesis
If the size of the aperture is small enough, the diffractiveeffects on the water wave become obvious (visible).
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength Variables
(a) Manipulated : Size of aperture or obstacle (b) Responding :Pattern of diffracted water waves (c) Fixed : Wavelength of water wave, frequency of
dipper and its distance from the aperture or obstacle
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1.4 Analysing Diffraction of Waves
(I) Fixed wavelength
Apparatus/Materials
Ripple tank, metal bars,mechanical stroboscope andpiece of white paper.
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1.4 Analysing Diffraction of Waves
(I) Fixed wavelength
Procedure
1 A ripple tank is filledwith water and set up asshown in Figure 1.41.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength
Procedure
2 Two pieces of metal barsare positioned to form a slitof width 10 cm at a distanceof 5 cm from the vibrating
wooden bar.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength Procedure
3 The dipper is switched onand the rheostat is adjusted sothat the wooden bar producesplane waves with awavelength of approximately4 cm.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength Procedure
4 The pattern of the wavesbefore and after passing the slitis observed and drawn.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength
Procedure
5 Steps 2 to 4 are repeated with different widths for
slits: 6 cm and2 cm respectively.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength Procedure
6 The two pieces of metal bars are replaced with asingle metal bar of length 10 cm placed centrallyat a distance of 5 cm from the vibrating wooden bar.
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1.4 Analysing Diffraction of Waves (I) Fixed wavelength Procedure
7 Steps 3 and 4 are repeated with differentlengths of metal bar: 6 cm and2 cmrespectively.
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1.4 Analysing Diffraction of Waves
(I) Fixed wavelength
Result(I) Fixed wavelength
(a) Small slit (b) Large slit (c) Large obstacle (d) Small obstacle
a, the effect
of diffraction is
very obvious.
< a, the effect
of diffraction is
not obvious.
The effect of
diffraction is not
obvious.
The effect of
diffraction is
obvious.
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1.4 Analysing Diffraction of Waves
Conclusion
As the size of the aperture or obstacle decreases,the effect of diffraction becomes obvious. Thehypothesis is accepted.
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1.4 Analysing Diffraction of Waves
(II) Fixed size of aperture
What is the relationship between the frequencyof a water wave and the effects of diffraction?
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1.4 Analysing Diffraction of Waves
(II) Fixed size of aperture
Hypothesis
The effects of diffraction of the water wavebecome more obvious as the frequency of the
wave decreases.
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1.4 Analysing Diffraction of Waves (II) Fixed size of aperture Variables
(a) Manipulated : Frequencyof dipper (i.e. frequency ofwater wave) (b) Responding : Pattern of diffracted waves (c) Fixed : Size of slit and its distance from the dipper
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1.4 Analysing Diffraction of Waves
(II) Fixed size of aperture
Procedure
1 A ripple tank is filled with water and set upas shown in Figure 1.41.
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1.4 Analysing Diffraction of Waves (II) Fixed size of aperture Procedure 2 Two pieces of metal bars are positioned to
form a slit of 5 cm wide 5 cm away from thevibrating wooden bar.
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1.4 Analysing Diffraction of Waves (II) Fixed size of aperture
Procedure
3 The vibrator is switched on. The frequency of the
water wave is gradually increasedby adjusting therheostat.
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1.4 Analysing Diffraction of Waves
(II) Fixed size of aperture
Procedure
4 The patterns of the waves passing throughthe slit at different frequencies are observed anddrawn.
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1.4 Analysing Diffraction of Waves (II) Fixed size of aperture
Results/Discussion
(II) Fixed size of slit
(a) Short
< a, the effect of
diffraction is not obvious.
(b) Long
a, the effect of
diffraction is very obvious.
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1.4 Analysing Diffraction of Waves (II) Fixed size of aperture
Conclusion
The effect of diffraction of a water wave became obviousas the frequency of the wave decreases.
The hypothesis is accepted
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1.4 Analysing Diffraction of Waves Example 9
In an experiment on the diffraction of water waves, a
dipper with a frequency of8 Hz produces waves with awavelength of2 cm. If the frequency of the dipper is 10Hz what is the wavelength of the water waves produced?
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1.4 Analysing Diffraction of Waves Example 9
Solution
Using the formula: where is constant (as the velocity of the wave is constant
in a ripple tank of uniform depth).
fv
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Diffraction of Light 1 Light is diffracted if it
passes through a narrow
slitcomparable in size toits wavelength. However,the effect is not obviousas the size of the slitincreases. This is becausethe wavelengths of lightare very short.
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Diffraction of Light
2. Diffraction of light is hardly noticeablecompared with diffraction of sound waves and
water waves because the wavelength of light isvery short(approximately 10-7 m).
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Diffraction of Light 3. Observation: (a) Narrow slit
(b) Wider slit
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Diffraction of Sound Waves
Experiment 1.7: To investigate the diffraction ofsound waves
Problem statement
Can a person hear the sound of a radio which ishidden behind an obstacle?
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Diffraction of Sound Waves
Hypothesis
The person can hear the sound of the radio
although it is behind an obstacle.
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Diffraction of Sound Waves Procedure 1 A radio is placed nearby a corner of a wall.
2 A student is requested to stand on the other side ofthe corner of the wall so that the radio is beyond hisvision.
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Diffraction of Sound Waves Procedure 3 The radio is turned on and the student is requested
to listen to the radio. 4 The position of the radio is changed and the effect
on the sound of the radio is listened to.
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Diffraction of Sound Waves Results
The student is able to hear the sound of the radio
although it is behind the wall (beyond his vision).
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Diffraction of Sound Waves Discussion
The sound of the radio spreads around the come the wall
due to diffraction of sound.
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Diffraction of Sound Waves Conclusion
The sound is able to spread
around a space or anobstacle. The hypothesis isvalid.
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