The Britannica Guide to Sound and Light (Physics Explained)
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Published in 2011 by Britannica Educational Publishing (a trademark of Encyclopædia Britannica, Inc.) in association with Rosen Educational Services, LLC 29 East 21st Street, New York, NY 10010. Copyright © 2011 Encyclopædia Britannica, Inc. Britannica, Encyclopædia Britannica, and the Thistle logo are registered trademarks of Encyclopædia Britannica, Inc. All rights reserved. Rosen Educational Services materials copyright © 2011 Rosen Educational Services, LLC. All rights reserved. Distributed exclusively by Rosen Educational Services. For a listing of additional Britannica Educational Publishing titles, call toll free (800) 237-9932. First Edition Britannica Educational Publishing Michael I. Levy: Executive Editor J.E. Luebering: Senior Manager Marilyn L. Barton: Senior Coordinator, Production Control Steven Bosco: Director, Editorial Technologies Lisa S. Braucher: Senior Producer and Data Editor Yvette Charboneau: Senior Copy Editor Kathy Nakamura: Manager, Media Acquisition Erik Gregersen: Associate Editor, Science and Technology Rosen Educational Services Nicholas Croce: Editor Nelson Sá: Art Director Cindy Reiman: Photography Manager Matthew Cauli: Designer, Cover Design Introduction by Jennifer Capuzzo Library of Congress Cataloging-in-Publication Data The Britannica guide to sound and light / edited by Erik Gregersen. p. cm. — (Physics explained) “In association with Britannica Educational Publishing, Rosen Educational Services.” Includes bibliographical references and index. ISBN 978-1-61530-374-8 (eook) 1. Sound—Popular works. 2. Light—Popular works. I. Gregersen, Erik. II. Title: Guide to sound and light. III. Title: Sound and light. QC225.3.G85 2011 534—dc22 2010013444 On the cover, p. iii: This graphic shows sound waves displayed on an oscilloscope. © www. istockphoto.com/Clearview images On page x: Prisms are able to break up white light into its component colors. Shutterstock.com On page xviii: This graphic shows sound waves displayed on an oscilloscope. Shutterstock.com On pages 1, 57, 69, 81, 147, 199, 223, 287, 332, 334, 336, 340: Shown here are sound waves represented on the screen of a device that measures the output of sound. Shutterstock.com CONTENTS 11 Introduction x Chapter 1: Sound 1 Plane Waves 2 Wavelength, Period, and Frequency 3 Amplitude and Intensity 4 The Decibel Scale 6 The Speed of Sound 7 Circular and Spherical Waves 11 Attenuation 13 Diffraction 15 31 Refraction 17 Refl ection 19 Impedance 20 Interference 23 The Doppler Effect 25 Sonic Booms 26 Standing Waves 27 In Stretched Strings 28 In Air Columns 33 In Solid Rods 37 31 In Nonharmonic Systems 38 Steady-State Waves and Spectral Analysis 41 42 The Sound Spectrograph 42 Generation by Musical Instruments 43 The Human Voice 45 Noise 46 Hearing 47 Dynamic Range of the Ear 47 The Ear as Spectrum Analyzer 50 Binaural Perception 52 Environmental Noise 53 Chapter 2: Acoustics 57 Early Experimentation 57 Modern Advances 60 Amplifying, Recording, and Reproducing 63 Architectural Acoustics 63 Reverberation Time 64 Acoustic Criteria 65 Acoustic Problems 67 Chapter 3: Beyond Human Hearing 69 89 Ultrasonics 69 Transducers 70 Applications in Research 71 95 Ranging and Navigating 72 The Doppler Effect 73 Materials Testing 73 High-Intensity Applications 74 Chemical and Electrical Uses 75 Medical Applications 75 Infrasonics 78 Chapter 4: Light 81 Theories of Light Through History 83 Ray Theories in the Ancient World 83 Early Particle and Wave Theories 84 Geometrical Optics: Light as Rays 87 102 Light Rays 87 Refl ection and Refraction 89 Total Internal Refl ection 93 Dispersion 94 Light as a Wave 97 Characteristics of Waves 98 Interference 100 Diffraction 106 Diffraction Effects 108 The Doppler Effect 112 Light as Electromagnetic Radiation 113 Electric and Magnetic Fields 113 Maxwell’s Equations 115 Electromagnetic Waves and the Electromagnetic Spectrum 115 The Speed of Light 119 Polarization 124 Energy Transport 130 Quantum Theory of Light 135 Blackbody Radiation 136 111 Photons 137 Quantum Mechanics 139 Emission and Absorption Processes 143 Quantum Electrodynamics 145 Chapter 5: Electromagnetic Radiation 147 General Considerations 147 Occurrence and Importance 148 The Electromagnetic Spectrum 149 Generation of Electromagnetic 118 Radiation 151 Properties and Behaviour 159 Cosmic Background Electromagnetic Radiation 165 Effect of Gravitation 166 The Greenhouse Effect of the Atmosphere 166 Forms of Electromagnetic Radiation 167 Radio Waves 168 Microwaves 173 Infrared Radiation 175 170 Visible Radiation 178 Ultraviolet Radiation 181 X-rays 183 204 Gamma Rays 195 Chapter 6: Lasers 199 History 199 Fundamental Principles 202 Energy Levels and Stimulated Emissions 203 Laser Elements 206 Laser Beam Characteristics 208 Types of Lasers 211 Laser Applications 212 Transmission and Processing of 246 Information 213 Precise Delivery of Energy 214 Alignment, Measurement, and Imaging 217 Chapter 7: Optics 223 Geometrical Optics 223 The Optical Image 224 Historical Background 224 Ray-Tracing Methods 229 Paraxial, or First-Order, Imagery 234 Optical Systems 241 Lens Aberrations 251 Image Brightness 258 Optics and Information Theory 263 General Observations 263 Image Formation 267 Partially Coherent Light 269 Optical Processing 273 Holography 280 Nonlinear Optics 286 255 Chapter 8: Biographies 287 Sir George Biddell Airy 287 Erasmus Bartholin 288 Nikolay G. Basov 289 Sir David Brewster 290 Arthur Holly Compton 291 Christian Doppler 293 Armand-Hippolyte-Louis Fizeau 293 Harvey Fletcher 295 Jean Foucault 296 Augustin-Jean Fresnel 296 294 Dennis Gabor 298 Hermann von Helmholtz 299 Christiaan Huygens 302 Edwin Herbert Land 307 Hans Lippershey 309 Theodore H. Maiman 309 Étienne-Louis Malus 310 A.A. Michelson 312 Sir Isaac Newton 315 Aleksandr Mikhaylovich Prokhorov 319 John William Strutt, 3rd Lord Rayleigh 320 Ole Rømer 324 303 Wilhelm Röntgen 326 Arthur L. Schawlow 328 Willebrord van Roijen Snell 329 327 Charles Townes 329 Thomas Young 330 Conclusion 332 Glossary 334 Bibliography 336 Index 340 INTRODUCTION 7 Introduction 7 he complex fields of light and sound may be rooted Tin complicated scientific theory, but their applica- tions are felt in everyday activities. The simple acts of watching television and grocery shopping have changed tremendously due to advancements in optics and acous- tics. Attending a concert can be more than just an entertaining experience when one considers the architec- tural, visual, and acoustical feats that all employ light and sound technology. The degree to which these branches of physics have impacted modern life is difficult to estimate, and the scientific exploration of their concepts continues to alter the world around us. Understanding sound waves is critical to anyone begin- ning to study sound. Plane waves spread through space as a plane rather than as a sphere of increasing radius. Plane waves, for their simplicity, are good models for clarifying the behaviour of sound waves in general and understand- ing terms such as wavelength, period, and frequency. While a plane wave travels through space in a linear fashion, more typically sound waves travel spherically, increasing in radius as they disseminate. These circular waves travel forward, creating new wave fronts. Not every waveform travels through the air as sound does. Two identical waves traveling on the same line in opposite directions form a standing wave. Both its shape and frequency match those of its two comprising waves. The bounded setting that helps produce a standing wave can be found in string and wind instruments. In a string instrument, for example, a string is stretched by fas- tening each end to a fixed point, and when the string is plucked, it vibrates and sends identical waves in opposite directions. Wind instruments act as bounded mediums for standing waves in a slightly different way. Instead of two separate waves composing the standing wave, it is the original wave driven into one end of the instrument that xi 7 The Britannica Guide to Sound and Light 7 reflects off the other end and travels in the opposite direc- tion of its original route that produces the standing wave. By applying the same principles to the human voice, it is easy to understand how different vowel sounds are formed. The vocal column is similar to a wind instru- ment in both structure and function. It is a closed-tube resonator, where one end, the vocal folds, is closed and the other end, the mouth, is open. The throat, mouth, and lips change shape, impacting the frequencies of the formants, determined by the tension in the vocal folds. The resulting variations in amplitude create different vowel sounds. Understanding how sound works is only part of the study of sound. Like the proverbial tree in the woods, there must be a receptor. As such, the ear’s range of response is huge. The ear canal acts as a closed tube, and the thresh- old of hearing varies depending on sound frequencies. For a low frequency, such as that of a heartbeat, the thresh- old for hearing rises, which explains why the human ear cannot continually detect such sounds without outside devices. For the same reason, audio components contain a feature that increases the intensity of low frequencies when the volume or loudness intensifies. The musical result equalizes the proportion of treble and bass to the ear, even as the loudness increases. The relationship between sound and hearing is analogous to the one between light and vision. Just as understanding how sound is created and disseminated is futile without the understanding of how the ear receives that information, a discussion of light behaviour needs to include some consideration of eye behaviour. It is that very relationship between the perception of light and the nature of light that perplexed the ancient world. The ancient Greek philosophers made the first doc- umented theories about the nature of light. Pythagoras, Empedocles, Epicurus, Euclid, and Ptolemy all proposed xii 7 Introduction 7 theories and conducted studies on how light and vision behaved. The nature of their studies differed in how each philosopher perceived the role of vision in the study of light. As a result, the human perception of light encum- bered the study of the early theories of light.