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Chapter1: Waves1.4 Analysing Diffraction of Waves
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1.4 Analysing Diffraction of Waves
• 1 Diffraction of waves is a phenomenon in which waves spread out as they pass through an aperture or 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 small enough,
• (b) the wavelength is large enough.
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1.4 Analysing Diffraction of Waves
• 3 Characteristics of diffracted waves:
• (a) Frequency, wavelength and speed of waves do not change.
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1.4 Analysing Diffraction of Waves
• 3 Characteristics of diffracted waves:
• (b) Changes in the direction of propagation and the pattern of the waves.
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1.4 Analysing Diffraction of Waves
• Experiment 1.6: to investigate the pattern of diffracted water waves
• (I) Fixed wavelength
• Problem statement
• What is the relationship between the size of the aperture or obstacle and the effects of diffraction 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 diffractive effects 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 and piece of white paper.
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1.4 Analysing Diffraction of Waves
• (I) Fixed wavelength
• Procedure
• 1 A ripple tank is filled with water and set up as shown in Figure 1.41.
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1.4 Analysing Diffraction of Waves
• (I) Fixed wavelength
• Procedure
• 2 Two pieces of metal bars are positioned to form a slit of width 10 cm at a distance of 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 on
and the rheostat is adjusted so that the wooden bar produces plane waves with a wavelength of approximately 4 cm.
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1.4 Analysing Diffraction of Waves
• (I) Fixed wavelength• Procedure• 4 The pattern of the waves
before and after passing the slit is 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 and 2 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 a
single metal bar of length 10 cm placed centrally at 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 different
lengths of metal bar: 6 cm and 2 cm respectively.
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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. The hypothesis is accepted.
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1.4 Analysing Diffraction of Waves
• (II) Fixed size of aperture
• What is the relationship between the frequency of 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 wave become 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 : Frequency of dipper (i.e. frequency of
water 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 up as 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 the vibrating 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 increased by adjusting the
rheostat.
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1.4 Analysing Diffraction of Waves
• (II) Fixed size of aperture
• Procedure
• 4 The patterns of the waves passing through the slit at different frequencies are observed and drawn.
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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 obvious as 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 of 8 Hz produces waves with a wavelength of 2 cm. If the frequency of the dipper is 10 Hz 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 slit comparable in size to its wavelength. However, the effect is not obviousas the size of the slit increases. This is because the wavelengths of light are very short.
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Diffraction of Light
• 2. Diffraction of light is hardly noticeable compared with diffraction of sound waves and water waves because the wavelength of light is very 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 of sound waves
• Problem statement
• Can a person hear the sound of a radio which is hidden 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 of
the corner of the wall so that the radio is beyond his vision.
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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 an obstacle. The hypothesis is valid.