Файл: Pye D. Polarised light in science and nature (IOP)(133s).pdf

Polarised Light in Science and Nature

Professor David Pye, born in 1932, was educated at Queen Elizabeth’s Grammar School, Mansfield, University College of Wales, 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 fascination for bat ‘radar’ and the electronic instrumentation necessary for the study of animal ultrasound. He was a Founder Director in 1976 of QMC Instruments Ltd, which produced large numbers of commercial ultrasound detectors, mainly for biological studies. He has travelled widely in order to study tropical bats and latterly has developed an interest in ultraviolet light and polarisation in the visual world of animals. A strong supporter of demonstration lectures, he gave the Royal 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 demonstration lecture which he calls ‘Polar Explorations—in Light’.

Polarised Light in

Science and Nature

David Pye

Emeritus Professor

Queen Mary, University of London

Institute of Physics Publishing

Bristol and Philadelphia

c IOP Publishing Ltd 2001

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 the Committee of ViceChancellors and Principals.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 0 7503 0673 4

Library of Congress Cataloging-in-Publication Data are available

Commissioning Editor: John Navas

Production Editor: Simon Laurenson

Production Control: Sarah Plenty

Cover Design: Victoria Le Billon

Marketing Executive: Colin Fenton

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 1035, 150 South Independence Mall West, Philadelphia, PA 19106, USA

Typeset in TEX using the IOP Bookmaker Macros

Printed in the UK by Hobbs the Printers, Totton, Hampshire

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 course of study but is essentially interdisciplinary, hoping to interest any enquiring mind: a reader taking any course or none at all. Such crosscultural influences appear to be deplorably unfashionable at present and this volume hopes to defend them by dealing with some simple unifying principles.

The book grew from a demonstration lecture, called ‘Polar Explorations in Light’ that I first developed for young audiences, initially at the Royal Institution of Great Britain. The 1874 classic book on polarised light by William Spottiswood also developed from a series of public lectures and I only hope that following such illustrious footsteps will achieve similar success. My own lecture has expanded to 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 elicit satisfying reactions from audiences of any age. It was gratifying, therefore, when the publishers suggested the possibility of a derivative book. I have tried to retain an element of the demonstration approach and, although no actual do-it-yourself-at-home recipes are given, I hope the descriptions are sufficiently helpful (and stimulating) to enable any resourceful reader to try things out. It is very rewarding to do and often quite easy, while many of the effects are much more beautiful than can be shown in photographs. Polaroid, as described in chapter 1, is widely available but if the larger sizes of sheet seem a little expensive, then the reflecting polarisers described in chapter 7 allow much to be done with the expenditure of nothing but a little ingenuity.

A reading list has been included in the hope that readers will want to find out more about some of the fields introduced here. This book does not attempt to be comprehensive in its treatment, simply to attract and intrigue. As always there is much to learn about a topic once you begin to get into it.

Acknowledgments

Several colleagues from Queen Mary, University of London have helped 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 Linda Humphreys and Lorna Mitchell (Library). Much encouragement 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), Chirotech Technology Ltd, Abercrombie and Kent Travel, Ernst Schudel (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, the Linnean Society Library, the Royal Society Library and Marie Odile Josephson of the Cultural Service at the French Embassy in London have all been enormously helpful, especially in tracing historical details.

Chapter 1

Aligning the waves

Polarised light is quite simply light in which the waves are all vibrating in one fixed direction. Most waves (sound waves are an exception) involve a vibration at right angles to their path. Waves on water go only up and down but the waves on a wiggled rope can be made to go up and down or from side to side or in any other direction around their line of travel. In just the same way, light waves can vibrate in any direction across their path. Now in ‘ordinary’ unpolarised light the direction of vibration is fluctuating rapidly, on a time scale of about 10 −8 s (a hundredth of 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 methods for showing that light is polarised and determining the direction of vibration will be described in this book, especially in chapters 2, 3 and 7.

An analogy with polarised light can be made by a wiggled rope that is passed through a narrow slit such as a vertical gap between fence posts or railings (figure 1.1). Vertical wiggles will pass unhindered through the slit but horizontal waves will be reduced or completely suppressed. If the rope is wiggled in all directions randomly, only the vertical components will pass through the slit. The equivalent effect with electromagnetic waves can be demonstrated with a low power microwave generator and detector (figure 1.2). Such waves, at a wavelength of 3 cm, are similar to those used in a microwave oven but in this case at less than a hundredthousandth of the power of an oven. Because of the way it works, the generator produces waves that vibrate in one direction only—polarised waves—and the detector is only sensitive to waves polarised in one

1

Figure 1.1. Waves pass along a wiggled rope. Where the rope passes through a slit in a fence, the waves continue if they are aligned with the slit but are stopped if they are transverse to the slit.

Figure 1.2. Apparatus to demonstrate polarisation with microwaves. A generator produces electromagnetic microwaves (3 cm wavelength radio waves) that vibrate in one direction only. A tuned receiver detects these waves only if they vibrate in one direction as shown by the deflection on a meter dial. With the two devices aligned, the meter is deflected, but detection ceases when either of them is twisted by 90◦ around their common axis. A grid with spacings less than the wavelength allows the waves to pass in one orientation but blocks them when it is turned onto its side around the axis of the beam.

direction. When the two are aligned, facing one another, the waves are detected as shown by the needle of a meter attached to the receiver, but if either unit is rotated onto its side, then reception ceases and the meter returns to zero although the waves are still being propagated.

With the generator and detector realigned and a signal being received, a wire grid with a spacing of about 7–8 mm (roughly onequarter of a wavelength) can be held across the beam. When the wires are in line with the direction of vibration, the beam is completely blocked, but rotating the grid by 90◦ restores full reception and the grid becomes completely ‘transparent’. (A grid aligned with the direction of vibration reflects the waves away, so blocking their path although one might expect this to be the orientation that allows them through.) It is easy to imagine that if the direction of vibration of the waves fluctuated randomly, then

the grid would block all the components with one direction and pass the rest, all vibrating in the other direction at right angles to the grid wires.

To be strict, these waves are known to consist of a vibration of the electrical field at right angles to an associated vibration of the magnetic field, hence the name electromagnetic waves. So there are actually two directions of vibration in any given wave. Most scientists and engineers assume ‘the’ vibration to be the electrical one and simply remember that the magnetic effect is there too, at right angles. Traditionally physicists did it the other way round, with ‘the’ vibration being the magnetic one, but nowadays this seems to be changing. Nevertheless one needs to check what convention any particular author is using. In common with almost universal current practice, this book refers to ‘the’ direction of vibration as that of the electric component. (Earlier texts referred to the ‘plane’ of polarisation and to ‘plane polarised’ light; for several good reasons these terms are now better replaced, as in this book, by the ‘direction’ of polarisation and ‘linearly polarised’ light respectively.)

Light waves are also electromagnetic waves, with exactly the same nature as microwaves except that the wavelength is about fifty thousand times smaller. The equivalent of wire grid polarisers can be made by embedding very fine arrays of parallel metallic whiskers in a thin transparent film; these are used at the rather longer infrared wavelengths and have also been made to work for light. But in general the short wavelengths of light require one to look for structures on the scale of atoms and molecules. Early studies of polarisation used crystals whose regular lattice of atoms can interact with light waves in some interesting ways, as described in chapter 3. Such devices were tricky to make and therefore expensive. They were also quite long and narrow, with a small area, or working aperture, or else they were of poor optical quality, which limited their use in optical instruments. In 1852 William Bird Herapath described a way of making thin crystals with strong polarising properties from a solution of iodine and quinine sulphate. Unfortunately these crystals, which came to be called herapathite, were so extremely delicate that their application was seldom practical, although Sir David Brewster did try some in his kaleidoscopes (see chapter 2).

Then, around 1930, Edwin Land developed ways of aligning microscopic crystals of herapathite while fixing them as a layer on a plastic sheet to make a thin, rugged polarising film that was soon called J-type polaroid. A series of developments followed rapidly and soon superseded the original material. H-type polaroid was made by absorbing iodine on a stretched sheet of polyvinyl alcohol. K-type

Figure 1.3. One polariser, a sheet of polaroid film, only allows half of the random, unpolarised light to pass but this is then all vibrating in one direction. Such polarised light passes easily through a second polaroid that is aligned with the first (left) but is completely blocked when the two polaroids are crossed (right). This simple but striking demonstration can easily be demonstrated to an audience on an overhead projector.

polaroid was made without iodine by stretching polyvinyl alcohol films and then dehydrating them. In a sense these materials resemble the wire grid used with microwaves, since the long polymer molecules are aligned by the stretching process. Both H-type and K-type are still much used, sometimes combined as HR-type polaroid which is effective for infrared waves. By adding dyes to the material, L-type polarisers were created that only polarised a part of the spectrum while freely transmitting the rest or, conversely, that transmitted only one colour. These materials soon found a very wide range of applications from components in scientific instruments to domestic sunglasses. Land always hoped that polaroid filters, with the direction of vibration set at 45 ◦, would become standard on car headlamps. Crossed polaroids in the windscreen or on glasses worn by the driver would then block the glare of oncoming traffic while being aligned with the car’s own lamps, so making only a small reduction in their effectiveness and even allowing them to remain undipped. Clearly this would only be helpful if every vehicle were so equipped and it has not come about.

The availability of polaroid has made observations of polarised light enormously more accessible as well as greatly increasing the applications of polarised light. For the highest optical quality, professionals still sometimes need to use expensive and inconvenient

Figure 1.4. A simple polariscope to detect polarisation can be made by two pieces of polaroid film with their polarisation directions at right angles to each other. When it is rotated against a background of polarised light, each half turns dark in turn but at the precise intermediate positions they are equally ‘grey’. Except in this latter state, the contrast between the two pieces is a more sensitive indicator than can be achieved by rotating a single piece to see if darkening occurs. An alternative arrangement with the polaroids at right angles is shown in colour plate 7.

Nicol prisms (see chapter 3) but polaroid is generally cheap, robust, thin and can be easily cut to any desired shape. It can be incorporated into cameras, microscopes and other instruments without any radical redesign or machining and it allows any amateur tremendous scope for exploiting the many properties of polarised light, which would have been inconceivable even to the specialist before 1930. The main disadvantage with polaroid is that, because it absorbs half the energy of the light, it can easily get very hot, especially if infrared ‘heat-rays’ are involved as with powerful filament lamps. It may be necessary to use a heat filter and/or a cooling fan in some cases.

A simple demonstration of the polarising action of polaroid, and also a test that it is polaroid rather then a simple tinted filter, is to overlap two pieces and rotate one (figure 1.3). When the polaroids are aligned, the light that passes through the first is also passed by the other. But when they are crossed, almost no light passes through both—they look black where they overlap. With tinted filters, of course, two always look darker than one and rotation makes no difference. The direction of polarisation for any given specimen of polaroid can easily be determined by looking through it at light reflected from a horizontal shiny surface such as gloss paint, varnish, water or glass. Such light is horizontally polarised, as described in detail in chapter 7. So when the polariser is turned to the vertical, the reflection appears to be dimmed or completely suppressed. A small mark can then be made in one corner of the polariser for future reference.

An instrument used to detect the presence of polarisation is called a polariscope. In its simplest form it is just a piece of polaroid or any other polariser that is rotated as a source of light is viewed through it. If the brightness of the source appears to vary with the rotation, then the light must be polarised. But this is often tedious and a slow fluctuation in brightness is not always easy to judge. It is much better to have two pieces of polaroid orientated at right angles and placed next to each other. A contrast in brightness can then be seen quickly and much more sensitively. If the direction of polarisation happens to be at exactly 45◦ to the two polariser directions then they will appear equally bright (figure 1.4) but this is unlikely to occur often and is easily eliminated by rocking the instrument slightly around its axis. Even better polariscopes will be described in the next chapter.