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Light and Dark.Pdf LIGHT AND DARK DAVID GREENE Institute of Physics Publishing Bristol and Philadelphia ­c IOP Publishing Ltd 2003 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher. Multiple copying is permitted in accordance with the terms of licences issued by the Copyright Licensing Agency under the terms of its agreement with Universities UK (UUK). British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN 0 7503 0874 5 Library of Congress Cataloging-in-Publication Data are available Commissioning Editor: Nicki Dennis Production Editor: Simon Laurenson Production Control: Sarah Plenty Cover Design: Fr´ed´erique Swist Marketing: Nicola Newey and Verity Cooke Published by Institute of Physics Publishing, wholly owned by The Institute of Physics, London Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS1 6BE, UK US Office: Institute of Physics Publishing, The Public Ledger Building, Suite 929, 150 South Independence Mall West, Philadelphia, PA 19106, USA Typeset in LATEX2ε by Text 2 Text, Torquay, Devon Printed in the UK by J W Arrowsmith Ltd, Bristol 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 LIGHT AND DARK 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 vi Contents 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 vii 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, zo- ology, 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 nav- igational information for sailors and flashes of almost incompre- hensible 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 for- mal 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 light- emitting 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 to meet 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 from the experience of most readers. There is information that could lead to more rational choices when buying sunglasses or light bulbs. ix LIGHT AND DARK Subjects like photonic crystals and polaritons are deemed too ab- 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 do not realize that the average time between sun- rise 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 rea- sons. I have included some easily demonstrated visual effects that were first noted in the 19th century but are rarely included in sci- ence courses. The human eye perceives colours in certain moving black-and-white patterns and has some ability to identify polar- ized 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 thou- sands of years light has been a carrier of messages, often for mili- tary 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 ma- terial. Chapter 1 makes no great demands on the reader. Chap- ters 2 and 3 are concerned with astronomical cycles involving the Sun and the Moon and form a basis for understanding the calen- dars described in chapter 4. Chapters 6 and 7 are mainly biologi- cal 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 po- larized light could also miss out chapter 5, which is about light in the sky. The use of light in human communications is described in x Preface chapters 8 and 10. Chapter 9 provides some technical and histori- cal background needed to appreciate the modern optical commu- nication 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 writ- 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 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 and many improvements, both literary and technical, thanks are due to Graham Saxby.
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