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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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(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)

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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’.