chem584.opticalproperties - Optical Properties of Materials...

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1 1 ISSUES TO ADDRESS. .. • What happens when light shines on a material? • Why do materials have characteristic colors? • Optical applications: luminescence photoconductivity solar cell optical communications fibers • Why are some materials transparent and others not? Optical Properties of Materials 2 Optical Properties Light has both particulate and wavelike properties: Photons hc h E m/s) 10 x (3.00 light of speed c ) s J 10 62 . 6 ( constant s Planck' frequency wavelength energy 8 34 x h E λ for visible light: 400 nm (blue) to 800 nm (red)
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2 3 Refractive Index, n Typical glasses ca. 1.5 -1.7 Plastics 1.3 -1.6 PbO (Litharge) 2.67 Diamond 2.41 medium) in light of (velocity vacuum) in light of (velocity v c • Transmitted light distorts electron clouds. Polarizable ions (e.g., lead ) are most effective in decreasing the speed of light. • Light is slower in a material vs vacuum. n = refractive index + no transmitted light transmitted light + electron cloud distorts • Note: n is a function of the wavelength of light 4 Total Internal Reflectance  sin sin n n n ’(low) n (high) n > n 1 c ' 1 reflected internally is light for 90 when occurs c i i c  angle critical angle refracted angle incident  c i i
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3 5 Example: Diamond in air Fiber optic cables are clad in low n material for this reason. 5 . 24 41 . 2 1 sin sin 1 sin 90 sin 1 41 . 2 sin sin  c c c c n n 6 Optical Fibers prepare preform by extrusion preform drawn to 125 m or less capillary fibers plastic cladding applied 60 m
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4 7 Optical Fiber Profiles Step-index Optical Fiber Graded-index Optical Fiber 8 • Incident light is either reflected, absorbed, or transmitted: S R A T o I I I I I Light Interaction with Solids • Optical classification of materials: single crystal polycrystalline dense polycrystalline porous Transparent Translucent Opaque Incident: I 0 Absorbed: I A Transmitted: I T Scattered: I S Reflected: I R
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5 9 Absorption of photons by electron transition: • Metals have a fine succession of energy states. Near-surface electrons absorb visible light. Optical Properties of Metals: Absorption Energy of electron I n c i d e t p h o Planck’s constant (6.63 x 10 -34 J/s) freq. of incident light filled states unfilled states E = h required! I o f r g y 10 Light Absorption t I I ln 0 t I I e 0 thickness sample cm ] [ t coefficien absorption linear 1 t Beer’s Law: A = ε l C
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6 11 Reflectivity = I R / I o is between 0.90 and 0.95. Reflected light is same frequency as incident. Metals appear reflective (shiny)!
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chem584.opticalproperties - Optical Properties of Materials...

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