©2016flippedaroundphysics.com

3.2LongitudinalandtransversewavesAsdiscussedin3.1,progressivewavestransferenergyfromoneplacetoanother.Thismeansthatthedirectionofwavetravelisthedirectiontheenergyismoved.Oscillationsthatfacilitatethismovementofenergyareeitherparalleltothisdirection(longitudinalwaves)oratrightanglestothisdirection(transversewaves).Itisbesttoobservesomeanimationsoflongitudinalandtransversewaves:http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.htmlSoundwavesandseismic-Pwavesareexamplesoflongitudinalwaves.Theyareoftendescribedascompressionalwavesasparticlesinthemediumaresqueezedtogether(compressed)andstretchedapart(calledrarefaction),asthewavepasses.Waterwaves,seismic-Swavesandelectromagneticwavesareexamplesoftransversewaves.Electromagneticwavesdonotrequireamediuminwhichtotravel.Inelectromagneticwavestheoscillationsareinthemagneticandelectricfieldswhicharenotonlyatrightanglestothedirectionofwavetravel,butatrightanglestoeachother:http://www.walter-fendt.de/html5/phen/electromagneticwave_en.htmAllelectromagneticwavestravelatthesamespeed(thespeedoflight)inavacuum.(1)!Whatisthespeedoflightinavacuum?(2)!Whathappenstoelectromagneticwaveswhentheyenteramaterial?PolarisationThefollowingdiagramshowsanelectromagneticwavethatisplane-polarised:

Theelectricfield(E)isoscillatingintheverticalplaneandthemagneticfield(B)isoscillatinginthehorizontalplane.Plane-polarisedmeansthateitherEandBareoscillatinginoneplaneonly.Onlytransversewavescanbepolarised.

©2016flippedaroundphysics.com

Thefollowingdiagramshowsacircularlypolarisedwave:

Inthiscaseonlytheelectricfieldorientationisshown.

Inthefollowingdiagram,randomlypolarisedelectromagnetic(EM)wavesarepassedthroughapolariser,whichonlyletsthroughacertaincomponent,producingplane-polarisedlight.Thetransmittedplanepolarisedwavehasalowerintensitythantherandomlyorientatedwave.

Thesituationabovecanbestudiedusingamicrowavesandametalgrilleforapolarizer.Whentheelectricfield(showninred)oscillatesinthedirectionofthemetalgrille,anoscillatingcurrentisproducedinthegrillewhichstopstransmissionofthewave.Ifthegrilleisatrightanglestotheelectricfield,nocurrentisproducedinthegrilleandthewavesistransmitted(asshown).(3)!Whentwopolarisersareangledat90°toeachother,thisisasituationknownas‘crossedpolarisers’.Whatdoyouthinkhappensinthissituation?(4)!Dosomeresearchandfind3usesofpolarisers.

randomlypolarisedwavepolariser

planepolarisedwave