IntrotoPolarizedLight:usingpolarizerstolookatmaterialsinadifferentlight.byTedKinsmanAssistantProfessorDepartmentofPhotographicSciencesRochesterInstituteofTechnologyFebruary,2017

Amazingcolorscanbecreatedinfrozenicecrystals.Hereicecrystalsbegintoformasthewaterfreezes.Thewaterwasplacedinaclearglasspetridishbetweentwopolarizingfilters.Photographedatabout1xmagnification.Polarizedlightislightthathaswavesoscillatingallinthesamedirection.Therearetwobasicwaystogetpolarizedlight,fromreflectionorbyusingafilter.Whenlightreflectsoffwateroranyotherflatsurfacethelightbecomespolarizedinonedirection.Itiscommonforfishermentousedpolarizedglassestoblockthereflectedpolarizedlightwhichcausesglaresotheycanseebelowthewaterbetter.Thesecondandmostcommonwaytoproducepolarizedlightiswithapolarizingfilter.PolarizingfilterswereinventedbyEdwinH.Landinthe1930’s.Landdevelopedaninexpensiveprocesstoalignpolarizing crystals and fix them into

position with a binder. This polarizing filter became the standard for photographic and industrial processes.

Two images of the identical scene. The left image has no polarizing filter, while the right image has a polarizing filter rotated to block the reflected light. Light reflected off the surface of water is strongly polarized, or vibrating in the same direction. To experiment with polarized light, you will need two polarizers, these can be easily purchased on the internet. There are also other sources for polarizers, like the glasses from a 3D movie or even an old pair of polarizing sunglasses. For best results, try to obtain two relatively large sheets 3 to 4 inches across. Manymaterialsshowinternalstresswhenilluminatedwithpolarizedlightandphotographedwithasecondanalyzingpolarizer.Thisuniquepropertyofmaterialsiscalledbirefringenceandiscausedbythepolarizedlightinteractingdifferentlywiththestressedmaterial,comparedtotheunstressedparts.Someofthemorecommonmaterialthatexhibitsthispropertyareglass,ice,plastic,andmanytypesofminerals.Thefulllistofmaterialswouldincludethousandsofentries.Toobservetheeffectsofpolarizedlightonamaterial,thematerialisplacedbetweentwosheetsofpolarizingmaterial.Thefirstfilteriscalledthepolarizerwhilethesecondfilteriscalledtheanalyzer.Ifthesecondfilterisplacedata90-degreeangletothefirst,thelightpassingthroughisalmostcompletelyeliminated.Thesecondsheetofpolarizingmaterialdoesnothavetobeincontactwiththesample;itcanbeplacedinfrontoftheobserver’seyeorinfrontofacamera.Withouttheanalyzer,youwouldnotbeabletoobservethebirefringenceinasample.Iliketothinkofthesesystemsasapolarizingsandwichwherethemeatistheobjectshowingthestress,whilethebreadrepresentsthepolarizers.

Equipmentsetup.P1isthefirstpolarizingsheet(polarizer),P2isthesecondpolarizer(analyzer)andonthefarrightisthecamera.Thesampleisplacedbetweenthetwopolarizingsheets.

Twopolarizersarealignedinthesamedirection.Thisorientationallowslighttotravel.Note:thecentersectionisjustalittledarker.

Herethesecondpolarizingfilterontherightisrotated90degreestotheoneontheleft.Thisorientationblocksmostofthelight.

Herethepolarizingfiltersareinthesameorientationasabove,butaninjectedplasticpetridishisplacedbetweenthetwofilters.Thestresscausedfromthemanufacturingprocessallowsdifferentcolorstopassthrough.Thisprocessiscalledbirefringenceandmostplasticswillexhibitthiseffect.

Gypsumcrystalsdemonstratebirefringence,herethedifferentthicknessesofthecrystalshowupasdifferentcolors.Therocksampleisplacedbetweentwopolarizingfiltersandisabout2incheswideinthisimage.

Formanyyears,mineralshavebeenidentifiedbythewaythinsectionsectionsofrockinteractwithlightinaspecialpolarizingmicroscope.Hereisasampleofhornblendephotographedat80xmagnification.

Aclose-upviewofthestresscreatedbyaC-clampwithpressureappliedtoablockofglass.

Asequenceoffourpunchesshownatdifferenttimings.Thepressurefromthepunchcreatesstressinaclearplasticgelwhichinturnexhibitsthebirefringence.Youcanseethepolarizingfilterinthebackground,whilethesecondfilterisinfrontofthelensonthecamera.

Akaratepunchisvisualizedinpolarizedlightwiththehelpofalargeblockofballisticgel.Thegelisbasicallyapolymerthatexhibitsbirefringence.Theforceofthepunchcanbeclearlyvisualizedwithpolarizedlighttoshowthestressinthegel;createdbythepunch.Thisballisticgeliscreatedtosimulatehumanflesh.

Sources:PolarizingSheetRosco Polarizing #7300 Filter - 17x20" Sheet http://www.bhphotovideo.comAlsoavailablefromnumeroussuppliersOnhttp://www.Amazon.com