Physics for Geologists, Second edition

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6 Electromagnetic radiation All radiation, whether we are considering the warmth given out from a fire, the light from a lamp, or the X-rays in a hospital, is electromagnetic by nature and propagates at the speed of light. Electromagnetic radiation behaves in some respects as waves, in others as particles. We categorize radiation by its frequency of oscillation (see Table 6.1) and/or by its wavelength. It must be remembered that there is no sharp dividing line between the categories - X-rays grade into y-rays, and their properties also merge, and radar waves (which can also heat, much as the microwave oven does) merge into radio waves. The subdivision is based on general characteristics, such as the visible part of the spectrum or the frequencies suitable <strong>for</strong> radio transmission. Frequency may be regarded as more fundamental than wavelength because a particle can oscillate with a frequency even if it does not behave as a wave; and frequency is independent of the medium through which the Table 6.1 Electromagnetic radiation - approximate frequencies and wavelengths y-rays X-rays Ultra-violet (UV) Visible light Infra-red (IR) Microwaves Radio waves Copyright 2002 by Richard E. Chapman Approximate values 100 x 10l8 10-l2 300 1015 800 x 10l2 370 x lop9 400 x 10l2 750 x lop9 10l2 300 x lo9 300 lo-3 10 lo3 30 lo3
66 Electromagnetic radiation radiation travels, whereas wavelength is not (as we saw when considering the refraction of light). Cosmic radiation Cosmic radiation, which was discovered early in the twentieth century, con- sists of very high frequency, high energy X-rays together with very high energy particles. The general level was found to be roughly constant over 24 hours, so clearly the radiation does not come from the Sun. The inten- sity was found, by apparatus flown in balloons, to increase up to a height of about 12 km (a height now flown by many aircraft!), then to decrease; so some cosmic radiation, or its effect, is clearly generated within the atmosphere. Cosmic radiation is much more intense in the polar regions than the equatorial, so clearly some of the particles are charged positively, others negatively. The equatorial regions are protected by the magnetic field's being approximately tangential to the Earth's surface near the Equator. Gamma rays y-rays are high-energy radiation from radioactivity with frequencies greater than 100 x 1018 Hz (wavelengths shorter than about 10-12 m, or 1 pm), and their significant property is that they are not affected by magnetic fields so they are not charged particles. a and 0-particles are affected by magnetic fields. y-rays are emitted spontaneously from the nucleus of some unstable isotopes. They are gradually absorbed when passing through solids, but can penetrate some millimetres of steel, some centimetres of concrete, and rather more centimetres of sedimentary rock. The main isotopes yielding y-rays are of the uranium and thorium families, and 4 0 ~ (potassium-40). 4 0 ~ occurs in the micas and K-feldspar, and clay minerals by cation-exchange. Mudrocks or mudstones have a level of y-radiation that is significantly higher than that of sandstones and most carbonates. The natural y-radiation is, there- <strong>for</strong>e, a useful borehole-logging parameter. When y-rays are absorbed by a nucleus, neutrons and a-particles are emitted. Catalytic activity is enhanced by y-rays. Most clay minerals act as catalysts in petroleum-chemical reactions, and most emit y-rays spontaneously, and this may be significant in the generation of petroleum. The catalytic cracking unit in petroleum refineries used to use natural Fuller's Earth (which contains montmorillonite and other clay minerals) as the catalyst to improve the yield of high-octane-number gasoline and reduce the gas yield. X-rays X-rays were discovered in 1895 by Wilhelm Rijntgen when he found fluo- rescence in a cathode tube when electrons were beamed from the cathode Copyright 2002 by Richard E. Chapman
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Physics for Geologists Second editi
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Copyright 2002 by Richard E. Chapma
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... vlii Contents Polarization 47 L
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Figures Copyright 2002 by Richard E
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Tables Copyright 2002 by Richard E.
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xiv Preface waves diminishes with d
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xvi Preface provides. This book is
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xviii Acknowledgements to K. Whaler
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xx Symbols mass refractive index bu
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2 Basic concepts Table 1.1 Dimensio
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4 Basic concepts Table 1.2 Basic SI
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6 Basic concepts pascal (Pa) and si
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8 Basic concepts all the relevant d
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10 Basic concepts relating them by
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2 Force The dimensions of force are
Page 31 and 32: 14 Force The ratio of the Moon's ac
Page 33 and 34: 16 Force Figure 2.1 The sum of the
Page 35 and 36: 18 Force When a motorcycle takes a
Page 37 and 38: 20 Force Figure 2.3 Torque is the p
Page 39 and 40: 22 Force How does momentum differ f
Page 41 and 42: 24 Force the weights - that is, the
Page 43 and 44: 26 Force Figure 2.6 Wall-sticking i
Page 45 and 46: 3 Gravity Universal gravity When di
Page 47 and 48: 30 Gravity are significant to geolo
Page 49 and 50: 32 Gravity Figure 3.2 The tractive,
Page 51 and 52: 34 Gravity Mean sea level It might
Page 53 and 54: 36 Gravity The benefits of space fl
Page 55 and 56: 38 Gravity Figure 3.6 Profile of th
Page 57 and 58: 40 Gravity Figure 3.8 The siphon at
Page 59 and 60: 4 Optics Light is an electromagneti
Page 61 and 62: 44 Optics Figure 4.2 Refraction. (a
Page 63 and 64: 46 Optics The coeficient of reflect
Page 65 and 66: 48 Optics only the light oscillatin
Page 67 and 68: 50 Optics called Iceland Spar. Two
Page 69 and 70: 52 Optics Diffraction If you shine
Page 71 and 72: 54 Optics Figure 4.7 Geometrical op
Page 73 and 74: 56 Optics Figure 4.9 The mirrors in
Page 75 and 76: 5 Atomic structure and age-dating P
Page 77 and 78: 60 Atomic structure and age-dating
Page 79 and 80: 62 Atomic structure and age-dating
Page 81: 64 Atomic structure and age-dating
Page 85 and 86: 68 Electromagnetic radiation Source
Page 87 and 88: Heat and heat flow The noun heat ha
Page 89 and 90: 72 Heat and heat flow Second Law: A
Page 91 and 92: 8 Electricity and magnetism Electri
Page 93 and 94: Electricity and magnetism 77 Figure
Page 95 and 96: Electricity and magnetism 79 rivett
Page 97 and 98: Electricity and magnetism 8 1 defin
Page 99 and 100: Electricity and magnetism 83 is kno
Page 101 and 102: 9 Stress and strain Pressure, p, is
Page 103 and 104: Stress and strain 87 than some natu
Page 105 and 106: These three elastic constants are r
Page 107 and 108: Stress and strain 91 Figure 9.1 New
Page 109 and 110: Stress and strain 93 Figure 9.2 Com
Page 111 and 112: Stress and strain 95 Figure 9.3 Ide
Page 113 and 114: Fracture Stress and strain 97 We fo
Page 115 and 116: Stress and strain 99 from which we
Page 117 and 118: 10 Sea waves The nature of water wa
Page 119 and 120: Sea waves 103 Figure 10.1 Wave moti
Page 121 and 122: Sea waves 105 refraction by islands
Page 123 and 124: Acoustics: sound and other waves 10
Page 125 and 126: Acoustics: sound and other waves 10
Page 127 and 128: 12 Fluids and fluid flow Introducti
Page 129 and 130: Fluids and fluid flow 1 13 Figure 1
Page 131 and 132: Fluids and fluid flow 115 Figure 22
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Fluids and fluid flow 117 and the f
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water above a vertical thickness h
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Fluids and fluid flow 121 where V i
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Fluids and fluid flow 123 ardentes,
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Fluids and fluid flow 125 Figure 12
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- 14 $ 12 E 10 K .- 3 8 r cll + 6 0
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Fluids and fluid flow 129 the liqui
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Some dangers of mathematical statis
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Appendix 139 What would happen if y
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Notes 141 and since x is very large
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References Allum, J. A. E. (1966) P
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References 145 -(1950) 'Mechanism o
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Further reading 147 Trigg, G. L. an
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Physics for Geologists, Second edition

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