InteractionsofWaves
Itisahot,sunnyday.Youarethefirstpersonto
enterthecalmwateroftheswimmingpool.Totestthe
temperatureofthewater,youdiponefootinfirst.Your
footcausesaseriesofripplestotravelacrossthewaterto
thefarwallofthepool.Aseachripplehitsthewall,it
bouncesoffthewallandtravelsbacktowardyou.
Reflection
Whenwater
waveshitthesideof
aswimmingpool,
theybounceback.
Whenanobjector
wavehitsasurface
thoughwhichit
cannotpass,it
bouncesback.Thisis
calledreflection.
Toshow
reflectionofawave,
drawalineto
representasurface.
Drawanotherlineto
showawavemoving
towardthesurfaceat
anangle.Noimaginealineperpendiculartothesurface.Theangelofincidenceistheanglebetween
theincomingwaveandtheimaginaryperpendicularline.Theangleofwaveandtheimaginary
perpendicularline.Theangleofreflectionistheanglebetweenthereflectedwaveandtheimaginary
line.Thelawofreflectionstatesthattheangleofreflectionequalstheangleofincidence.Allwave
obeythelawofreflection.
Therearemanyexamplesofreflectioninyoureverydaylife.Aballthathitsawallbounces
back,orisreflected.Whenyoulookinamirror,youusereflectedlighttoseeyourself.Anechoisan
exampleofreflectedsound.
Refraction
Perhapsyouhavetriedtograbasinkingobjectwhenyouareinaswimmingpool,onlytocome
upempty-handed.Yetyouweresureyougrabbedrightwhereyousawtheobject.Youmissed
grabbingtheobjectbecausethelightraysfromtheobjectchangeddirectionastheypassedfromthe
waterintotheair.Thebendingofwavesduetoachangeinspeediscalledrefraction.
Diffraction
Sometimes
wavescanbend
aroundanobstacle
intheirpath.For
example,wavescan
passthrougha
narrowentrancetoa
harborandthen
spreadoutsidethe
harbor.Figure10
showswaterwaves
diffractingasthey
enteraharbor.
Whenawavepassesabarrierormovesthroughaholdinabarrier,itbendsandspreadsout.
Thebendingofwavesaroundtheedgeofabarrierisknownasdiffraction.Figure11showsawater
wavepassingthroughaholdinabarrierandanotherbendingaroundabarrier.Ineachcase,yousee
thewavediffractingontheothersideofthebarrier.
Interference
Supposethatyouandafriendareeachholdingoneendofarope.Ifyoubothflicktheendsat
thesametime,yousendtwowavestowardseachother.Whatwillhappenwhenthosetwowaves
meet?
Whentwoormorewavesmeet,theyhaveaneffectoneachother.Thisinteractioniscalled
interference.Therearetwotypesofinterference:constructiveanddestructive.
ConstructiveInterferenceConstructiveinterferenceoccurswhenevertwowavescombinetomakea
wavewithalargeramplitude.Youcanthinkofconstructiveinterferenceaswaves“helpingeach
other”togiveastrongerresult,orcombiningenergy.
Figure12Ashowstwoidenticalwaves(sameamplitude,samewavelength)travelinginthe
samedirectionatthesametime.Ifthetwowavestravelalongthesamepathatthesametimethey
willbehaveasone.Whatwillthecombinedwavelooklike?Thecrestsofthefirswavewilloccurat
thesameplaceasthecrestofthesecondwave.Theenergyfromthetwowaveswillcombine.Thus
theamplitudeofthenewwavewillbetwicetheamplitudeofeitheroftheoriginalwaves.
Ifthewaves
havethesame
wavelengthbut
different
amplitudes,the
crestwillstill
occuratthesame
placeandadd
together.The
resulting
amplitudewillbe
thesumofthetwo
original
amplitudes.
Similarly,the
troughswilloccur
together,makinga
deepertroughthan
eitherwavealone.
Destructive
Interference
Whenthe
amplitudesoftwowavescombinewitheachotherproducingasmalleramplitude,theresultiscalled
destructiveinterference.Whathappensifthecrestdon’tmeetatthesameplace?Inthiscase,one
wavecomesaftertheother.Figure12Bshowswhathappenswhenthecrestsofthefirstwaveoccurat
thesameplaceasthetroughsofthesecondwave.Theamplitudeofthefirstwavecancelsoutthe
amplitudeofthesecondwave.Thistypeofinterferenceproducesawavewithamplitudeofzero.The
originalwavesseemtobedestroyed.Ifthetwowaveshavedifferentamplitudes,theywillnotcancel
eachotheroutbutwillcombinetoproduceawavewithasmalleramplitude.
Twoidenticalwavescantravelalongthesamepath,onealittlebehindtheother.Whenthis
happens,thewavescombineconstructivelyinsomeplacesanddestructivelyinothers.