CONTENTS
PREFACE ix
1 ESSENTIAL, USEFUL AND FRIVOLOUS LIGHT 1
1.1 Light for life 1
1.2 Wonder and worship 4
1.3 Artificial illumination 6
1.3.1 Light from combustion 6
1.3.2 Arc lamps and filament lamps 9
1.3.3 Gas discharge lamps 13
1.4 Light in art and entertainment 15
2 PATTERNS OF SUNLIGHT 19
2.1 The year 19
2.2 Equinoxes and eccentricity 22
2.3 The length of a day 26
2.4 The length of daylight 31
2.5 The length of a second 37
3 MONTHS AND MOONLIGHT 41
3.1 The lunar month and the lunar orbit 41
3.2 The lunar nodes and their rotation 43
3.3 The lunar day 48
3.4 The length of moonlight 50
3.5 Eclipses and Saros cycles 55
3.5.1 Eclipses and history 55
3.5.2 The Saros cycle 55
3.5.3 Total and annular solar eclipses 61
3.6 Tides 63
4 HISTORY, DATES AND TIMES 67
4.1 Solar calendars 67
4.2 The Roman Catholic Church and the development
of astronomy 70
4.3 The start of the year 72
4.4 Lunar and other calendars 73
4.5 Time zones 77
4.6 The International Date Line 80
5 LIGHT AND THE ATMOSPHERE 84
5.1 Scattered light and twilight 84
5.2 Polarization of light 88
5.3 Rainbows 94
5.4 Cloudy skies 98
5.5 Halos 99
6 SEEING THE LIGHT 103
6.1 The human eye 103
6.2 Colour vision and colour blindness 109
6.2.1 Colour vision 109
6.2.2 Colour blindness 112
6.3 Polarization sensitivity 114
6.4 Speed of response 115
6.5 Optical illusions 118
7 ZOOLOGICAL DIVERSIONS 129
7.1 Colour vision in animals 129
7.2 Zebras 131
7.2.1 Species and subspecies 131
7.2.2 Other zebra-striped animals 133
7.3 Piebald coats and unusual goats 134
7.4 Jellicle cats are black and white 138
7.5 Cephalopods 142
7.6 Lighting up for a mate or a meal 144
7.6.1 Bioluminescence in insects 144
7.6.2 Bioluminescence in deep-sea fish 146
7.7 More anatomical oddities 147
8 INFORMATION IN LIGHT 150
8.1 Lighthouses 150
8.2 Semaphores for optical telegraphy 154
8.3 Morse, Mance and the heliograph 163
8.4 Bell and the photophone 166
9 LIGHT IN THE ERA OF ELECTRONICS 170
9.1 Electronics 1900–1960 170
9.1.1 Early rectification devices 170
9.1.2 The solid-state rectifier 172
9.1.3 The transistor 175
9.2 New semiconductors for optoelectronics 177
9.3 Optoelectronic semiconductor devices 182
9.4 Bright light from cool solids 187
10 OPTICAL COMMUNICATION TODAY 193
10.1 Waveguides and optical fibres 193
10.2 The transparency of glass 195
10.3 Optical fibres 198
10.4 Optical amplification 203
10.5 Conveying sound by light 204
10.6 The long and the short of optical communication 210
BIBLIOGRAPHY 213
INDEX 215
PREFACE
In December 2001 Martin Creed was awarded the Turner Prize
worth £20 000 for a work of contemporary art entitled ‘The Lights
Going On and Off’. It consists of an empty room with its most
conspicuous feature aptly described by its title. Clearly a book
on a similar theme is timely, though unlikely to be so financially
rewarding.
This book brings together a wide range of topics that would
normally be found in separate texts classified as astronomy, zoology,
technology, history, art or physics. The connection is the
theme of light and dark, which may alternate either in time or in
space. In the time domain, slow variations often determine when
animals mate and sleep, patterns defined in seconds provide navigational
information for sailors and flashes of almost incomprehensible
brevity convey messages and data around the world.
Spatial patterns in black and white define the area on which chess
players compete and enable the computer at the supermarket
checkout to distinguish baked beans from jam tarts.
This book is intended to provide entertainment as well as
instruction, and is in no way a comprehensive textbook for formal
courses. For some more detailed accounts of particular topics
you should refer to the suggestions for further reading. I have
also mentioned places where you can look at such things as lightemitting
fish and military heliographs. I have carefully avoided
any mathematical analysis, but assume that readers will not be
terrified by information presented in diagrams and graphs. Some
parts of the book may be useful to students reluctantly following
a science course tomeet requirements for a broad curriculum. The
topics reflect some quirks of my own personality and history, but
generally they have been chosen because they are not far fromthe
experience of most readers. There is information that could lead
to more rational choices when buying sunglasses or light bulbs.
Subjects like photonic crystals and polaritons are deemed too abstruse
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 interesting
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 do not realize that the average time between sunrise
and sunset is about twelve and a quarter hours in Britain on
21st March and 21st September. I hope that readers will not only
take notice of this apparent paradox but also understand the reasons.
I have included some easily demonstrated visual effects that
were first noted in the 19th century but are rarely included in science
courses. The human eye perceives colours in certain moving
black-and-white patterns and has some ability to identify polarized
light.
Patterns of light and dark are not always natural phenomena
to be observed and enjoyed. Human ingenuity allows them to be
created for entertainment or for conveying information. For thousands
of years light has been a carrier of messages, often for military
and naval purposes. In the 19th century army signallers used
sunlight to send messages in less than a minute across distances
that took a horseman a day. By the middle of the 20th century,
copper wires and radio waves seemed to have captured most of
the market in rapid long-distance communication. Nevertheless
fifty years later incredibly short flashes of infrared light convey
huge amounts of data and speech from continent to continent at
an extraordinarily low cost.
It is not essential to read all the chapters in strict numerical
order, but some of them do require acquaintance with earlier material.
Chapter 1 makes no great demands on the reader. Chapters
2 and 3 are concerned with astronomical cycles involving the
Sun and the Moon and form a basis for understanding the calendars
described in chapter 4. Chapters 6 and 7 are mainly biological
and do not require any knowledge of the contents of chapters
2, 3 and 4. Readers seeking information about vision and light
emission in the animal kingdom and already familiar with polarized
light could also miss out chapter 5, which is about light in
chapters 8 and 10. Chapter 9 provides some technical and historical
the sky. The use of light in human communications is described in
background needed to appreciate the modern optical communication
systems described in chapter 10. These last three chapters
are best read in sequence but could be tackled without reading
any of the first seven.
Numerous people have made helpful inputs during the writing
of this book. Nick Lovibond at the Australian Antarctic Division,
Michael Land at the University of Sussex and S Krebs at
the Schweizerischer Ziegenzuchtverband kindly supplied information
to a total stranger. Clare McFarlane, Steve Oliver and
Sue Wheeler commented constructively on various chapters. Jane
Greene produced some of the drawings and read the whole text
critically more than once. For the final careful review of the entire
book andmany improvements, both literary and technical, thanks
are due to Graham Saxby.
Figures 1.6, 5.12, 5.16 and 7.10 are reproduced by kind permission
of the National Gallery, ClareMcFarlane, Kip Ladage and
Oxford Scientific Film, respectively.
David Greene
Harlow
March 20
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