WhatAreWaves?
Farouttosea,thewinddisturbsthecalmsurfaceofthewater.Arippleforms.Asthewind
continuestoblow,theripplegrowsintoapowerfulwavethatcantravelagreatdistance.Nearthebeach,
surferswaiteagerly.Theyquicklypaddleintodeeperwatertocatchthemonstrouswave.Surfersenjoy
thepowerofnatureastheyridethewavetotheshore.
Whatarewaves?Howcantheytravelsofar?Whyaresomewavesmorepowerfulthanothers?In
thissection,youwillexplorehowwavesbeginandhowtheymove.
WavesandEnergy
Wavescrashingonabeachshowthetremendousenergywaves
cancarry.Awaveisdisturbancethattransfersenergyfromplaceto
place.Inscience,energyisdefinedastheabilitytodowork.To
understandwaves,thinkofaboatoutontheocean.Ifawavedisturbsthe
surfaceofthewater,itwillcauseanythingfloatingonthewatertobe
disturbed,too.Theenergycarriedbyawavecanliftevenalargeshipasit
passes.Thedisturbancecausedbyawaveistemporary.Afterthewave
haspassed,thewateriscalmagain.
WhatCarriesWaves?Manywavesrequiresomethingtotravelthrough.
Waterwavestravelalongthesurfaceofthewater,andsoundwaves
travelthroughair.Youcanevenmakeawavetravelalongarope.The
materialthroughwhichawavetravelsiscalledamedium.Gases(suchas
air),liquids(suchaswater),andsolids(suchasropes)allactasmediums.
Wavesthatrequireamediumthroughwhichtotravelarecalled
mechanicalwaves.
Althoughwavestravelthroughamedium,theydonotcarrythe
mediumitselfwiththem.LookattheduckinFigure1.Whenawaves
movesundertheduck,theduckmovesupanddown.Itdoesnotmovealongthesurfaceofthewater.
Afterthewavespasses,thewaterandtheduckreturntowheretheystarted.Breakingwavesatabeach
behavealittledifferently.Whenwaveshitabeach,thewaterdoesmovealongwiththewave.This
happensbecausetheoceanfloornearthebeachslopesupward.Asthewateratthebottomofthewave
hitstheslope,itmovesuptowardthetopofthewave.Thetopofthewavegetsbiggerandcontinuesto
moveforward.Eventuallyittopplesover,turningwhiteandfrothy.
Notallwavesrequireamediumtocarrythemalong.Lightfromthesun,forexample,cantravel
throughemptyspace.Lightisanexampleofanelectromagneticwave.
WhatCausesWaves?Youcancreatewavesbydippingyourfingerinwater.Wavesarecreatedwhena
sourceofenergycauseamediumtovibrate.Avibrationisarepeatedbackandforthorupanddown
motion.Thismotionisthewave.
TypesofWaves
Differenttypesofwavestravelthroughmediumsindifferentways.Wavesareclassifiedaccording
tohowtheymove.Thethreetypesofwavesaretransversewaves,longitudinalwavesandsurfacewaves.
TransverseWaves:
Whenyoumakeawaveonarope,thewavemovesfromoneendoftheropetotheother.The
ropeitself,however,movesupanddownorfromsidetoside.Wavesthatmovethemediumatright
anglestothedirectioninwhichthewavesaretravelingarecalledtransversewaves.Transversemeans
“across.”Asatransversewavemovesinonedirection,theparticlesofthemediummoveacrossthe
directionofthewave.Figure2showsthatsome
partsoftheropeareveryhighwhilesomeare
verylow.Thehighestpartofthewaveiscalleda
crest,andthelowestpartsarecalledtroughs.
LongitudinalWaves.Figure3showsadifferent
kindofwave.Ifyoustretchoutaspringtoyand
pushandpulloneend,youcanproducea
longitudinalwave.Longitudinalwavesmovethe
particlesofthemediumparalleltothedirection
inwhichthewavesaretravel.Thecoilsinthe
springmovebackandforthparalleltothe
wavemotion.
NoticeinFigure3thatinsomepartsof
thespringthecoilsareclosetogether.Inother
parts,thecoilsaremorespreadout.Theparts
wherethecoilsareclosetogetherarecalled
compression.Thepartswherethecoilsare
spreadout,orrarefied,arecalledrarefactions.
Ascompressionsandrarefactionstravel
alongthespringtoy,eachcoilmovesslightly
forwardandthenback.Theenergytravelsfromoneendofthespringtotheother,creatingawave.After
thewavepasses,eachpartofthespringreturnstothepositionwhereitstarted.
Combinationsofwaves:
Surfacewavesarecombinationsoftransverseandlongitudinalwaves.Thesewavesoccuratthe
surfacebetweentwomediums,suchaswaterandair.Whenawavepassesthroughwater,thewater
(andanythingonit)movesupanddown,likeatransversewaveonarope.Thewateralsomovesback
andforthslightlyinthedirectionthatthewaveistraveling,likethecoilsofthespring.Butunlikethe
coilsofaspring,waterdoesnotcompress.Theupanddownandbackandforthmovementscombineto
makeeachparticleofwatermoveinacircle.Figure4showsthecircularmotionofsurfacewaves.