Light and Dark

230 Pages·2002·1.987 MB·English

by  David Greene

L
IGHT AND
D
ARK
DAVID GREENE
Institute ofPhysics Publishing
Bristol andPhiladelphia
c IOPPublishingLtd2003
(cid:1)
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CONTENTS
PREFACE ix
1 ESSENTIAL,USEFULANDFRIVOLOUSLIGHT 1
1.1 Lightforlife 1
1.2 Wonderandworship 4
1.3 Artificialillumination 6
1.3.1 Lightfromcombustion 6
1.3.2 Arclampsandfilamentlamps 9
1.3.3 Gasdischargelamps 13
1.4 Lightinartandentertainment 15
2 PATTERNSOFSUNLIGHT 19
2.1 Theyear 19
2.2 Equinoxesandeccentricity 22
2.3 Thelengthofaday 26
2.4 Thelengthofdaylight 31
2.5 Thelengthofasecond 37
3 MONTHSANDMOONLIGHT 41
3.1 Thelunarmonthandthelunarorbit 41
3.2 Thelunarnodesandtheirrotation 43
3.3 Thelunarday 48
3.4 Thelengthofmoonlight 50
3.5 EclipsesandSaroscycles 55
3.5.1 Eclipsesandhistory 55
3.5.2 TheSaroscycle 55
3.5.3 Totalandannularsolareclipses 61
3.6 Tides 63
4 HISTORY,DATESANDTIMES 67
4.1 Solarcalendars 67
4.2 TheRomanCatholicChurchandthedevelopment
ofastronomy 70
LIGHTANDDARK
4.3 Thestartoftheyear 72
4.4 Lunarandothercalendars 73
4.5 Timezones 77
4.6 TheInternationalDateLine 80
5 LIGHTANDTHEATMOSPHERE 84
5.1 Scatteredlightandtwilight 84
5.2 Polarizationoflight 88
5.3 Rainbows 94
5.4 Cloudyskies 98
5.5 Halos 99
6 SEEINGTHELIGHT 103
6.1 Thehumaneye 103
6.2 Colourvisionandcolourblindness 109
6.2.1 Colourvision 109
6.2.2 Colourblindness 112
6.3 Polarizationsensitivity 114
6.4 Speedofresponse 115
6.5 Opticalillusions 118
7 ZOOLOGICALDIVERSIONS 129
7.1 Colourvisioninanimals 129
7.2 Zebras 131
7.2.1 Speciesandsubspecies 131
7.2.2 Otherzebra-stripedanimals 133
7.3 Piebaldcoatsandunusualgoats 134
7.4 Jelliclecatsareblackandwhite 138
7.5 Cephalopods 142
7.6 Lightingupforamateorameal 144
7.6.1 Bioluminescenceininsects 144
7.6.2 Bioluminescenceindeep-seafish 146
7.7 Moreanatomicaloddities 147
8 INFORMATIONINLIGHT 150
8.1 Lighthouses 150
8.2 Semaphoresforopticaltelegraphy 154
8.3 Morse,Manceandtheheliograph 163
8.4 Bellandthephotophone 166
vi
Contents
9 LIGHTINTHEERAOFELECTRONICS 170
9.1 Electronics1900–1960 170
9.1.1 Earlyrectificationdevices 170
9.1.2 Thesolid-staterectifier 172
9.1.3 Thetransistor 175
9.2 Newsemiconductorsforoptoelectronics 177
9.3 Optoelectronicsemiconductordevices 182
9.4 Brightlightfromcoolsolids 187
10 OPTICALCOMMUNICATIONTODAY 193
10.1 Waveguidesandopticalfibres 193
10.2 Thetransparencyofglass 195
10.3 Opticalfibres 198
10.4 Opticalamplification 203
10.5 Conveyingsoundbylight 204
10.6 Thelongandtheshortofopticalcommunication 210
BIBLIOGRAPHY 213
INDEX 215
vii
PREFACE
In December 2001 Martin Creed was awarded the Turner Prize
worth£20000foraworkofcontemporaryartentitled‘TheLights
Going On and Off’. It consists of an empty room with its most
conspicuous feature aptly described by its title. Clearly a book
on asimilar theme istimely, though unlikelyto beso financially
rewarding.
Thisbookbringstogetherawiderangeoftopicsthatwould
normally be found in separate texts classified as astronomy, zo-
ology, technology, history, art or physics. The connection is the
themeoflightanddark,whichmayalternateeitherintimeorin
space. Inthetimedomain,slowvariationsoftendeterminewhen
animalsmateandsleep,patternsdefinedinsecondsprovidenav-
igationalinformation for sailorsand flashesof almost incompre-
hensible brevity convey messages and data around the world.
Spatialpatternsinblackandwhitedefinetheareaonwhichchess
players compete and enable the computer at the supermarket
checkouttodistinguishbakedbeansfromjamtarts.
This book is intended to provide entertainment as well as
instruction, and is in no way a comprehensive textbook for for-
malcourses. Forsomemoredetailedaccountsofparticulartopics
you should refer to the suggestions for further reading. I have
alsomentionedplaceswhereyoucanlookatsuchthingsaslight-
emitting fish and military heliographs. I have carefullyavoided
any mathematical analysis, but assume that readers will not be
terrifiedbyinformationpresentedindiagramsandgraphs. Some
partsofthebookmaybeusefultostudentsreluctantlyfollowing
asciencecoursetomeetrequirementsforabroadcurriculum.The
topicsreflectsomequirksofmyownpersonalityandhistory,but
generallytheyhavebeenchosenbecausetheyarenotfarfromthe
experienceof mostreaders. Thereisinformation thatcould lead
to more rational choices when buying sunglasses or light bulbs.
ix
LIGHTANDDARK
Subjectslikephotoniccrystalsandpolaritonsaredeemedtooab-
struse for inclusion, even though they are fascinating topics for
scientists currently investigating interactions between light and
matter.
Although zebras and rainbows are familiar, they have in-
teresting features that frequently pass unnoticed. An aim of
this book is to encourage the reader to look more carefully at
such sights. Many people are aware of the spring and autumn
equinoxes but donot realizethatthe averagetime betweensun-
riseandsunsetisabouttwelveandaquarterhoursinBritainon
21stMarchand21stSeptember. Ihopethatreaderswillnotonly
takenoticeofthisapparentparadoxbutalsounderstandtherea-
sons. Ihaveincludedsomeeasilydemonstratedvisualeffectsthat
werefirstnotedinthe19thcenturybutarerarelyincludedinsci-
encecourses. Thehumaneyeperceivescoloursincertainmoving
black-and-white patterns and has some ability to identify polar-
izedlight.
Patternsoflightanddarkarenotalwaysnaturalphenomena
tobeobservedandenjoyed. Humaningenuityallowsthemtobe
createdforentertainmentorforconveyinginformation. Forthou-
sandsofyearslighthasbeenacarrierofmessages,oftenformili-
taryandnavalpurposes.Inthe19thcenturyarmysignallersused
sunlight to send messages in less thana minute acrossdistances
that took a horseman a day. By the middle of the 20th century,
copper wiresand radio wavesseemed to have capturedmost of
the market in rapid long-distance communication. Nevertheless
fifty years later incredibly short flashes of infrared light convey
huge amounts of dataand speechfromcontinent to continent at
anextraordinarilylowcost.
It is not essential to read all the chapters in strict numerical
order,butsomeofthemdorequireacquaintancewithearlierma-
terial. Chapter 1 makes no greatdemands on the reader. Chap-
ters2and3areconcernedwithastronomicalcyclesinvolvingthe
SunandtheMoonandformabasisforunderstandingthecalen-
darsdescribedinchapter4. Chapters6and7aremainlybiologi-
calanddonotrequireanyknowledgeofthecontentsofchapters
2, 3 and 4. Readers seeking information about vision and light
emission in the animal kingdom and already familiar with po-
larizedlightcouldalsomissoutchapter5,whichisaboutlightin
thesky.Theuseoflightinhumancommunicationsisdescribedin
x
Preface
chapters8and10. Chapter9providessometechnicalandhistori-
calbackgroundneededtoappreciatethemodernopticalcommu-
nicationsystemsdescribedinchapter10.Theselastthreechapters
are best read in sequence but could be tackled without reading
anyofthefirstseven.
Numerouspeoplehavemadehelpfulinputsduringthewrit-
ing of this book. Nick Lovibond at the Australian Antarctic Di-
vision, Michael Land at the University of Sussex and S Krebs at
the Schweizerischer Ziegenzuchtverband kindly supplied infor-
mation to a total stranger. Clare McFarlane, Steve Oliver and
SueWheelercommentedconstructivelyonvariouschapters.Jane
Greene produced some of the drawings and readthe whole text
criticallymorethanonce. Forthefinalcarefulreviewoftheentire
bookandmanyimprovements,bothliteraryandtechnical,thanks
areduetoGrahamSaxby.
Figures 1.6, 5.12, 5.16 and 7.10 are reproduced by kind per-
missionoftheNationalGallery,ClareMcFarlane,KipLadageand
OxfordScientificFilm,respectively.
DavidGreene
Harlow
March2002
xi
1
ESSENTIAL, USEFUL AND
FRIVOLOUS LIGHT
1.1 Light forlife
Life on Earth is almost totally dependent on the regular input
of energy that is supplied by radiation from the Sun. The input
maintains the temperature of most of the sea and the land sur-
facewithinarangethatallowslivingcreaturestofunction. Some
of the sunlight provides the energy for photosynthesis, the pro-
cessplantsuse to convertcarbondioxide andwater into oxygen
andcarbohydratessuchasglucose. Theproductsofphotosynthe-
siscontainmoreenergythanthestartingmaterials,andotherlife
forms, such as animals and fungi, can exploit the stored energy.
Theanimalsinhaletheoxygenandconsumetheplants,eitherdi-
rectly (herbivores) or indirectly (carnivores), and return carbon
dioxideandwatertotheenvironment.
Photosynthesisisamultistagechemicalprocessinwhichthe
keyroleisplayedbychlorophyll. Thereareseveralsubtlydiffer-
ent forms of this complex organic compound, but the molecules
ofallformscontainjustoneatomofmagnesium. Chlorophyllob-
tains the energy necessary for the synthesis of carbohydrates by
selectivelyabsorbinglightfrombothendsofthevisiblespectrum,
asshowninfigure1.1.Lightinthemiddleofthevisiblespectrum
isnotabsorbedbutreflected,sotheleavesofmostplantsappear
green. Whereas colours within the visible spectrum have wave-
lengths from 370 to 740 nanometres (nm) (1 nm 10(cid:1)9 m) and
(cid:1)
areeitherbeneficialorharmless,ultravioletlighthasadestructive
1