Lec 23 - ENGR ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 ENGR 4250 – Advanced Materials Engineering Lecture 23 Chapter 21: Optical Properties
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Why is this phenomenon occuring?
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Chapter 21: Optical Properties SUES TO ADDRESS ISSUES TO ADDRESS. .. What happens when light shines on a material ? Why do materials have characteristic colors? Why are some materials transparent and others not? Optical applications: -- luminescence -- hotoconductivity photoconductivity -- solar cell -- optical communications fibers
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Electromagnetic Radiation s m c / 10 3 1 8 × = = = λυ In vacuum 0 0 ∈ μ
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Electromagnetic Radiation
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Optical Properties Light has both particulate and wavelike properties hotons - ith mass Photons with mass = ν = Δ hc h E λ energy = Δ E frequency wavelength = ν = λ m/s) 10 x (3.00 light of speed c ) s J 10 62 . 6 ( constant s Planck' 8 34 = = x h
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Phonon vs Photon Phonon imilarity Photon imilarity Similarity Wave like uantized Similarity Wave like uantized Quantized issimilarity Quantized issimilarity Dissimilarity Elastic solids Dissimilarity Electromagnetic Many media Velocity sound Velocity light
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ATOMIC AND ELECTRONIC INTERACTIONS
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Electronic Polarization adiation is absorbed 1 Radiation is absorbed 2 Light waves are retarded in velocity
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Refractive Index, n Transmitted light distorts electron clouds. + no transmitted ht transmitted ht + electron cloud distorts Light is slower in a material vs vacuum. light light medium) in light of (velocity vacuum) in light of (velocity v c n = refractive index Note: n = f ( λ ) Typical glasses ca. 1.5 -1.7 --Adding large, heavy ions (e.g., lead can decrease the speed of light. --Light can be Plastics 1.3 -1.6 PbO (Litharge) 2.67 Diamond 2.41 "bent" Selected values from Table 21.1, Callister 7e .
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Total Internal Reflectance n > n φ sin n n ’(low) igh) ' 1 φ φ sin n n (high) angle incident = φ c φ angle critical angle refracted = φ = φ i i 1 φ c 90 when occurs i c ° = φ φ reflected internally is light for c i φ > φ
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Example: Diamond in air o 0 in 1 in sin 1 sin 90 sin 1 41 . 2 sin sin φ = φ = φ φ = c c n n o 5 . 24 1 1 sin = φ = φ c c 41 . 2 • Fiber optic cables are clad in low n material for this reason.
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Electron Transitions
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Light Interaction with Solids Incident light is either reflected, absorbed, or transmitted: S R A T o I I I I I + + + = Absorbed: I A ransmitted: Reflected : I R Incident: I 0 Transmitted: I T Scattered: I S Optical classification of materials: Adapted from Fig. 21.10, Callister 6e . (Fig. 21.10 is by J. Telford, Transparent Translucent with specimen preparation by P.A. Lessing.) Opaque single crystal polycrystalline dense polycrystalline porous
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LIGHT INTERACTIONS WITH SOLIDS I I I I + + = 0 Intensity I T T = I A A = I R R = 1 = + + R A T 0 I 0 I 0 I Transparent, Translucent, Opaque
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Metals
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Optical Properties of Metals: bsorption Absorption of photons by electron transition: Absorption Energy of electron unfilled states Δ E = h ν required!
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Lec 23 - ENGR ENGR 4250 Advanced Materials Engineering...

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