1.4 Diffraction

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Chapter1: Waves1.4 Analysing Diffraction of Waves



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.




• 2 The effect of diffraction is obvious only if

• a! the size of the aperture or obstacle is smallenough"

• b! the wavelength is large enough.




• # $haracteristics of diffracted waves%

• a! &re'uency" wavelength and speed of wavesdo not change.




• # $haracteristics of diffracted waves%

• b! $hanges in the directionof propagation and thepattern of the waves.




• Experiment 1.6: to investigate the pattern ofdiffracted water waves

• (I) Fixed wavelength

• Problem statement

• hat is the relationship between the si!e of the

apert"re or obstacle and the effects ofdiffraction of water waves#



1.4 Analysing Diffraction of Waves• (I) Fixed wavelength

• $%pothesis

• If the si!e of the apert"re is small eno"gh& the diffractiveeffects on the water wave become obvio"s (visible).



1.4 Analysing Diffraction of Waves• (I) Fixed wavelength• 'ariables

• (a) anip"lated : i!e of apert"re or obstacle• (b) *esponding : Pattern of diffracted water waves• (c) Fixed : avelength of water wave& fre+"enc% of

dipper and its distance from the apert"re or obstacle





• ,pparat"s-aterials

• *ipple tan& metal bars&mechanical stroboscope and piece of white paper.





• Proced"re

• 1 , ripple tan is filledwith water and set "p asshown in Fig"re 1./1.




• Proced"re

• 0 wo pieces of metal barsare positioned to form a slitof width 12 cm at a distanceof 3 cm from the vibrating

wooden bar.



1.4 Analysing Diffraction of Waves• (I) Fixed wavelength• Proced"re

• 4 he dipper is switched onand the rheostat is ad5"sted sothat the wooden bar prod"ces plane waves with awavelength of approximatel%/ cm.




• / he pattern of the wavesbefore and after passing the slitis observed and drawn.




• Proced"re

• 3 teps 0 to / are repeated with different widths for

slits: 6 cm and 0 cm respectivel%.




• 6 he two pieces of metal bars are replaced with asingle metal bar of length 12 cm placed centrall%at a distance of 3 cm from the vibrating wooden bar.




• (I) Fixed wavelength• Proced"re

• teps 4 and / are repeated with differentlengths of metal bar: 6 cm and 0 cmrespectivel%.





• *es"lt (I) Fixed wavelength

(a) Small slit (b) Large slit (c) Large obstacle (d) Small obstacle

λ ≥ a, the effect

of diffraction isvery obvious.

λ < a, the effect

of diffraction isnot obvious.

he effect ofdiffraction is notobvious.

he effect ofdiffraction isobvious.




• $onclusion

• As the si(e of the aperture or obstacle decreases"the effect of diffraction becomes obvious. Thehypothesis is accepted.




• ))! &i*ed si(e of aperture

• What is the relationship between the fre'uencyof a water wave and the effects of diffraction+





• ,ypothesis

• The effects of diffraction of the water wave become more obvious as the fre'uency of the

wave decreases.



1.4 Analysing Diffraction of Waves• ))! &i*ed si(e of aperture• -ariables

• a! anipulated % &re'uency of dipper i.e. fre'uency of water wave!

• b! /esponding % 0attern of diffracted waves• c! &i*ed % i(e of slit and its distance from the dipper





• 0rocedure

• 1 A ripple tan is filled with water and set up asshown in &igure 1.41.




• ))! &i*ed si(e of aperture• 0rocedure

• 2 Two pieces of metal bars are positioned toform a slit of 3 cm wide 3 cm away from the vibrating wooden bar.



1.4 Analysing Diffraction of Waves• ))! &i*ed si(e of aperture

• 0rocedure

• # The vibrator is switched on. The fre'uency of the

water wave is gradually increased by adusting the

rheostat.





• 0rocedure

• 4 The patterns of the waves passing throughthe slit at different fre'uencies are observed anddrawn.




• /esults5Discussion

(II) Fixed si!e of slit

(a) Short λ

λ < a, the effect of

diffraction is not obvious.

(b) Long λ

λ ≥ a, the effect of

diffraction is very obvious.




• $onclusion

• The effect of diffraction of a water wave became obviousas the fre'uency of the wave decreases.

• The hypothesis is accepted



1.4 Analysing Diffraction of Waves• 6*ample 7

• )n an e*periment on the diffraction of water waves" a

dipper with a fre'uency of 8 ,( produces waves with a wavelength of 2 cm. )f the fre'uency of the dipper is 19,( what is the wavelength of the water waves produced+



1.4 Analysing Diffraction of Waves• 6*ample 7

• olution

• :sing the formula%• where is constant as the velocity of the wave is constant

in a ripple tan of uniform depth!.

•

λ f v =



Diffraction of Light• 1 ;ight is diffracted if it

passes through a narrow

slit comparable in si(e toits wavelength. ,owever"the effect is not obvious as the si(e of the slit

increases. This is becausethe wavelengths of lightare very short.



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 appro*imately 19<= m!.



Diffraction of Light• #. >bservation%•

• a! ?arrow slit

• b! Wider slit



Diffraction of Sound Waves• Experiment 1.: o investigate the diffraction of

so"nd waves

• 0roblem statement

• $an a person hear the sound of a radio which ishidden behind an obstacle+



Diffraction of Sound Waves• ,ypothesis

• The person can hear the sound of the radioalthough it is behind an obstacle.



Diffraction of Sound Waves• Proced"re• 1 , radio is placed nearb% a corner of a wall.

• 0 , st"dent is re+"ested to stand on the other side ofthe corner of the wall so that the radio is be%ond hisvision.



Diffraction of Sound Waves• Proced"re• 4 he radio is t"rned on and the st"dent is re+"ested

to listen to the radio.• / he position of the radio is changed and the effecton the so"nd of the radio is listened to.



Diffraction of Sound Waves• /esults

• The student is able to hear the sound of the radio

although it is behind the wall beyond his vision!.



Diffraction of Sound Waves• Discussion

• The sound of the radio spreads around the come the wall

due to diffraction of sound.



Diffraction of Sound Waves• $onclusion

• The sound is able to spread

around a space or anobstacle. The hypothesis is valid.

1.4 Diffraction

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