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© 2016 flippedaroundphysics.com
3.4.1Waveinterference
Whenwavesofthesamefrequencyandsimilaramplitudeoverlap,theycombine(‘superpose’)toproduceareasofconstructiveinterferenceanddestructiveinterference.Constructiveinterferenceoccurswhenwavesfromdifferentsourcesarriveatacertainpointwithzerophasedifference.Thismeansthatpeaksarrivetogetherleadingto‘superpeaks’andtroughsarrivetogetherproducing‘supertroughs’.Destructiveinterferenceoccurswhenwavesfromdifferentsourcesarriveinantiphase(180°phasedifference).Thismeansthatapeakfromonesourcewillcancelatroughfromanothersourceandviceversa.
PhETSIM
Openthefollowingsimulation:
https://goo.gl/KS4NxaChoosethe interferencesimulation.
Selectthehighestfrequencyusingtheslider.
Youshouldseethepattern,below:
Wecanseeareasofconstructiveinterferenceanddestructiveinterference.
(1)!Drawredlinestoshowareasofconstructiveinterference.
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© 2016 flippedaroundphysics.com
(2)!Drawbluelinestoshowareasofdestructiveinterference.
Trychangingthedripseparation.
(3)!Whathappenstotheseparationoftheareasofconstructiveanddestructiveinterferenceasthedripseparationisincreased?
Pathdifference
Constructiveinterferenceoccursatapoint,ifthepathdifferencebetweensourcesofwavesismultiplesofwholewavelengths.
LightInterference
Nowchoosetheslits simulation.
Andcarefullyselectthethesettingsshowntotheright:
Movethebarrier(withslits)tothelefthandside.
S1
S2 We see that constructive interference occurs at P, because the path length S1 is exactly 1 wavelength longer than path length S2.
(4)! How would path length compare if we had a point where destructive interference was happening?
P
wave source (e.g. drips)
© 2016 flippedaroundphysics.com
Youshouldseesomethinglikethefollowing:
Thegapsinthebarrieractassourcesofwaves(similartothedrips).Theyspreadoutandinterfere,producinglightanddarkbands(fringes)onthescreen.
(4)!Nowchangetheslidertoselectredlight.Howdoesthisaffectthetheseparationofbrightfringesonthescreen?
(5)!Whatistheroughwavelengthofredlight(seevalueinscaleatthetopleftofthescreen)?
(6)!Intermsofmagnitude,howdoestheslitwidthcomparetothewavelengthofthelight?
Areas of high intensity where waves have combined constructively. Called ‘bright fringes’.
Areas of low intensity where waves have combined destructively. Called ‘dark fringes’.
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