ch19_optical - Chapter 19 Optical Properties c19cof01...

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c19cof01 c19cof01 Chapter 19: Optical Properties Refraction & Dispersion
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ISSUES TO ADDRESS... What happens when light shines on a material ? 1 Why do materials have characteristic colors? Optical applications: --luminescence --photoconductivity --solar cell --optical communications fibers Why are some materials transparent and others not?
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c19f01 c19f01 An electromagnetic wave Electric field and magnetic field components, and the wavelength .
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c19f02 c19f02 Figure 19.2 The spectrum of electromagnetic radiation, 3.1 eV 1.8 eV 2.2 eV 2.7 eV
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Watt/m 2 Dividing by I o Transparent, translucent and opaque Electronic Polarization: absorption & refraction (retarded waves) Electron Transitions: Quantum behavior, excited and ground states
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2 Incident light is either reflected, absorbed, or transmitted: Incident: I o Reflected : I R Absorbed : I A Transmitted : I T    I o = I T + I A + I R Optical classification of materials: Transparent Transluscent Opaque Adapted from Fig. 21.10, Callister 6e . (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.) LIGHT INTERACTION WITH SOLIDS
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c19f03 c19f03 Example: Isolated atom
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c19f04 Figure 19.4 ( a) Schematic representation of the mechanism of photon absorption for metallic materials in which an electron is excited into a higher-energy unoccupied state. The change in energy of the electron E is equal to the energy of the photon. ( b) Reemission of a photon of light by the direct transition of an electron from a high to a low energy state.
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3 Absorption of photons by electron transition: Metals have a fine succession of energy states. Near-surface electrons absorb visible light. Energy of electron Incident photon Planck’s constant (6.63 x 10 -34 J/s) freq. of incident light filled states unfilled states E = h ν required! I o of energy h ν Adapted from Fig. 21.4(a), Callister 6e . OPTICAL PROPERTIES OF METALS: ABSORPTION J s
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4 Electron transition emits a photon. Reflectivity = I R /I o is between 0.90 and 0.95. Reflected light is same frequency as incident. Metals appear reflective (shiny)! Adapted from Fig. 21.4(b), Callister 6e . OPTICAL PROPERTIES OF METALS: REFLECTION Energy of electron filled states unfilled states E I R “conducting” electron re-emitted photon from material surface
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c19tf01 c19tf01
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7 Transmitted light distorts electron clouds.
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