Waves 1.4 Diffraction

7/30/2019 Waves 1.4 Diffraction

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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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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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3 Characteristics of diffracted waves:

(a) Frequency, wavelength and speed of wavesdo not change.


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3 Characteristics of diffractedwaves:

(b) Changes in the directionof propagationand thepattern of the waves.


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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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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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Apparatus/Materials

Ripple tank, metal bars,mechanical stroboscope andpiece of white paper.


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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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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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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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7 Steps 3 and 4 are repeated with differentlengths of metal bar: 6 cm and2 cmrespectively.


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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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Conclusion

As the size of the aperture or obstacle decreases,the effect of diffraction becomes obvious. Thehypothesis is accepted.


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(II) Fixed size of aperture

What is the relationship between the frequencyof a water wave and the effects of diffraction?


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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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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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Procedure

4 The patterns of the waves passing throughthe slit at different frequencies are observed anddrawn.


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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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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.

Waves 1.4 Diffraction

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