Polarised Light in Science and Nature ProfessorDavidPye, bornin1932,was educatedat QueenElizabeth’s GrammarSchool,Mansfield,UniversityCollegeofWales,Aberystwyth and Bedford College for Women, London. He was lecturer and then reader at King’s College and has been Professor of Zoology at Queen Mary, University of London since 1973. He developed an early fascinationforbat ‘radar’andthe electronicinstrumentationnecessary for the study of animal ultrasound. He was a Founder Director in 1976 of QMC Instruments Ltd, which produced large numbers of commercialultrasounddetectors,mainlyforbiologicalstudies. Hehas travelledwidelyinordertostudytropicalbatsandlatterlyhasdeveloped an interest in ultraviolet light and polarisation in the visual world of animals.Astrongsupporterofdemonstrationlectures,hegavetheRoyal Institution Christmas Lectures in 1985, and shares the Dodo’s opinion that ‘the best way to explain it is to do it’. This book arose from a demonstrationlecturewhichhecalls‘PolarExplorations—inLight’. Polarised Light in Science and Nature David Pye Emeritus Professor Queen Mary, University of London Institute of Physics Publishing Bristol and Philadelphia c IOPPublishingLtd2001 (cid:13) All rights reserved. No part of this publication may be reproduced, storedinaretrievalsystemortransmittedinanyformorbyanymeans, electronic, mechanical, photocopying, recording or otherwise, without the priorpermissionofthe publisher. Multiplecopyingis permittedin accordancewiththetermsoflicencesissuedbytheCopyrightLicensing Agencyunderthe terms of its agreementwith the Committee of Vice- ChancellorsandPrincipals. BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary. ISBN0750306734 LibraryofCongressCataloging-in-PublicationDataareavailable CommissioningEditor:JohnNavas ProductionEditor:SimonLaurenson ProductionControl:SarahPlenty CoverDesign:VictoriaLeBillon MarketingExecutive:ColinFenton Published by Institute of Physics Publishing, wholly owned by The InstituteofPhysics,London InstituteofPhysicsPublishing,DiracHouse,TempleBack,BristolBS1 6BE,UK USOffice: InstituteofPhysicsPublishing,ThePublicLedgerBuilding, Suite1035,150SouthIndependenceMallWest,Philadelphia,PA19106, USA TypesetinTEXusingtheIOPBookmakerMacros PrintedintheUKbyHobbsthePrinters,Totton,Hampshire Contents Preface vii 1 Aligningthewaves 1 2 Changingdirection 7 3 Crystals 20 4 Fields 39 5 Lefthand,righthand 46 6 Scattering 60 7 Reflection 71 8 Goingcircular 87 9 Seeingthepolarisation 102 Somerecommendationsforfurtherreading 119 Index 121 Preface We humans cannot see when light is polarised and this leads us to unfortunatemisapprehensionsaboutit.Evenscientistswhoshouldknow better,oftenassumethatpolarisedlightisanobscuretopicofspecialised interest in only a few rather isolated areas; in fact it is a universal feature of our world and most of the natural light that we see is at least partially polarised. In the Animal Kingdom, insects and many otheranimalsexploitsuchnaturalpolarisationinsomefascinatingways since they do not share this human limitation and can both detect and analyse polarisation. It may be our unfamiliarity with this aspect of light that also makes many people think it is a ‘difficult’ subject, yet thebasisisextremelysimple.Whensuchmisconceptionsareovercome, the phenomena associated with polarisation are found to be important throughout science and technology—in natural history, and biology, geology and mineralogy, chemistry, biochemistry and pharmacology, physics and astronomy and several branches of engineering, including structural design, communications, high speed photography and sugar refining, as well as crafts such as glassblowing and jewellery. They also involve some very beautiful effects, most of which are easy to demonstrateandmanipulate. Ourgeneralunawarenessofwhatwearemissingisindeedagreat pity. This book hopes to put all this right and enrich its readers’ perception of the world. A small degree of repetition and overlap has seemed necessary in order to make each topic complete; I hope it does not become trying. The text deliberately uses no maths and onlytheminimumoftechnicalterms—itishopedthatrejectingjargon, however precise and convenient it may be to the specialist, will make the stories more accessible to the newcomer. In any case, the book covers such a wide range of science that each chapter would need a separatevocabularytobeintroducedanddefined,whichwouldbecome vii viii Preface tedious and might well deter many readers. Descriptive terms or even circumlocutions are sometimes quicker in the end. In any case this is not a textbook; it does not aim to help directly with any particular courseofstudybutisessentiallyinterdisciplinary,hopingtointerestany enquiringmind: a readertakinganycourseornoneat all. Suchcross- culturalinfluencesappeartobedeplorablyunfashionableatpresentand thisvolumehopestodefendthembydealingwithsomesimpleunifying principles. The book grew from a demonstration lecture, called ‘Polar ExplorationsinLight’thatIfirstdevelopedforyoungaudiences,initially at the Royal Institution of Great Britain. The 1874 classic book on polarisedlightbyWilliamSpottiswoodalsodevelopedfromaseriesof publiclecturesandIonlyhopethatfollowingsuchillustriousfootsteps will achievesimilar success. My ownlecturehas expandedto become a show that can now be adapted to almost any kind of audience. I was greatly drawn to the subject precisely because it brings in such a wide variety of phenomena across science, and because it allows one to perform some extremely beautiful demonstrations that never fail to elicitsatisfyingreactionsfromaudiencesofanyage. Itwasgratifying, therefore, whenthe publisherssuggestedthe possibility of a derivative book. I have tried to retain an element of the demonstrationapproach and,althoughnoactualdo-it-yourself-at-homerecipesaregiven,Ihope thedescriptionsaresufficientlyhelpful(andstimulating)toenableany resourcefulreadertotrythingsout. Itisveryrewardingtodoandoften quiteeasy,whilemanyoftheeffectsaremuchmorebeautifulthancan beshowninphotographs. Polaroid,asdescribedinchapter1,iswidely availablebutifthelargersizesofsheetseemalittleexpensive,thenthe reflectingpolarisersdescribedinchapter7allowmuchtobedonewith theexpenditureofnothingbutalittleingenuity. Areadinglisthasbeenincludedinthehopethatreaderswillwant to find out more about some of the fields introduced here. This book doesnotattempttobecomprehensiveinitstreatment,simplytoattract and intrigue. As always thereis muchto learn abouta topic once you begintogetintoit. Acknowledgments SeveralcolleaguesfromQueenMary,UniversityofLondonhavehelped me to develop some of the demonstrations used in the lectures. Ray Crundwell (Media Services) was solely responsible for processing Preface ix the photographs presented here and gave much invaluable advice. Others who have been especially helpful and have contributed in many different ways to the emergence of this book include Isaac Abrahams, Gerry Moss and Stuart Adams (Chemistry), Bill French and Kevin Schrapel (Earth Sciences), Edward Oliver (Geography), John Cowley (Glass Workshop), David Bacon (Media Services) and LindaHumphreysandLornaMitchell(Library). Muchencouragement and/or material help have been generously provided by Sir Michael Berry, Ken Edwards, Ilya Eigenbrot, Cyril Isenberg, Mick Flinn, Ken Sharples (Sharples Stress Engineering Ltd), Frank James and Bipin Parma (the Royal Institution of Great Britain), Dick Vane-Wright and Malcolm Kerley (Entomology Department, Natural History Museum), ChirotechTechnologyLtd,AbercrombieandKentTravel,ErnstSchudel (Photo-Suisse, Grindelwald, Switzerland), Murray Cockman (Atomic Weapons Research Establishment), Michael Downs (National Physical Laboratory), Jørgen Jensen (Skodsborg, Denmark), Søren Thirslund (Helsingor, Denmark), Hillar Aben (Estonian Academy of Science, Tallinn) and Brian Griffin (Optical Filters Ltd). The British Library, theLinneanSocietyLibrary,theRoyalSocietyLibraryandMarieOdile JosephsonoftheCulturalServiceattheFrenchEmbassyinLondonhave allbeenenormouslyhelpful,especiallyintracinghistoricaldetails. Chapter 1 Aligning the waves Polarisedlightisquitesimplylightinwhichthewavesareallvibratingin onefixeddirection.Mostwaves(soundwavesareanexception)involve avibrationatrightanglestotheirpath. Wavesonwatergoonlyupand downbutthewavesonawiggledropecanbemadetogoupanddown orfromsidetosideorinanyotherdirectionaroundtheirlineoftravel. In just the same way, light waves can vibrate in any direction across theirpath.Nowin‘ordinary’unpolarisedlightthedirectionofvibration is fluctuating rapidly, on a time scale of about 10−8 s (a hundredthof a millionth of a second), and randomly through all possible directions around the path of the ray. Polarisation simply consists of forcing the waves to vibrate in a single, constant direction. A number of simple methodsforshowingthatlightispolarisedanddeterminingthedirection of vibration will be described in this book, especially in chapters 2, 3 and7. Ananalogywithpolarisedlightcanbemadebyawiggledropethat ispassedthroughanarrowslitsuchasaverticalgapbetweenfenceposts orrailings(figure1.1).Verticalwiggleswillpassunhinderedthroughthe slitbuthorizontalwaveswillbereducedorcompletelysuppressed.Ifthe ropeiswiggledinalldirectionsrandomly,onlytheverticalcomponents will pass through the slit. The equivalent effect with electromagnetic wavescanbedemonstratedwithalowpowermicrowavegeneratorand detector(figure1.2).Suchwaves,atawavelengthof3cm,aresimilarto thoseusedinamicrowaveovenbutinthiscaseatlessthanahundred- thousandthof the powerof an oven. Because of the way it works, the generatorproduceswaves thatvibrateinonedirectiononly—polarised waves—and the detector is only sensitive to waves polarised in one 1