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21.1 - Chapter 21 Optical Properties ISSUES TO ADDRESS What...

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Unformatted text preview: Chapter 21: Optical Properties ISSUES TO ADDRESS... What happens when light shines onto a material? Why do some materials have characteristic colors? Why are some materials transparent and others not? Optical applications: o o o o luminescence photoconductivity solar cells optical communications fibers Chapter 21 - 1 ANNOUNCEMENTS Download: Web Chapter 21 (& Ch 20) from BbVista Reading: Chapter 21, All Sections Week 10 Recitations: No Quiz… Problems: 21.4, 21.14, 21.16, 21.18, 21.19, 21.23, 21.D1 Review items for the Final Exam, Monday, June 7 Final Exam: Friday, June 11, 1:00-3:00 pm Accommodations: LeBow 348; 1:00-4:00 (1.5X); 1:00-5:00 Noon (2X) Chapter 21 - 2 ANNOUNCEMENTS Final Exam Coverage: Chapters 17, 18, 20, 21 + some topics from previous chapters/earlier material… Style: Like Midterms…1 x multipart, short answer/circle the answer question + 5 recitation problem style questions, plus one 15-point (~9%) bonus Q Keys to Success: Read the textbook & posted lecture notes Read your own lecture notes Practice solving problems…Recitation, Example & other Take advantage of TA and RI office hours Chapter 21 - 3 ANNOUNCEMENTS Course Evaluations: Open…soon! Close, 2-3 weeks later Please complete an evaluation of the course, the recitation instructors…maybe after the final exam etc. o Your feedback provides us with useful information which we use to make changes in the future Chapter 21 - 4 Electromagnetic Radiation Considered as wave-like: electric (E) & magnetic field (H) components mutually perpendicular and also to direction of propagation – Poynting Vector, S = E X H (vector cross-product) light, heat, radar, radio waves, x-rays etc. em spectrum: – 10-12 m ( rays) to 105 m (radio waves) Chapter 21 - 5 The Electromagnetic Spectrum Visible Light: v. narrow range… 0.4-0.7 m (400-700 nm) perceived color = fn.( ) Chapter 21 - 6 “White” light = mix of all colors Electromagnetic Radiation All em radiation: traverses vacuum at same velocity c, 3 x 108 m/s c related to electric permittivity of vacuum and magnetic permeability of vacuum: 1 c= 0μ 0 also c = Sometimes more convenient to consider em radiation from quantum-mechanical viewpoint: packets (“photons”) of energy rather than waves photon energy E quantized…can only have discrete values… Chapter 21 - 7 Optical Properties Light has both particulate and wave-like properties – Photons…with mass – Photon energy E =h = hc = energy (J) = wavelength (m) = frequency (Hz) h = Planck’s constant (6.62 x 10-34 J.s) c = speed of light (3 x 108 m/s) Chapter 21 - 8 Interaction of Light with Solids • Incident light is either Transmitted, Absorbed, Reflected or Scattered: i.e. I = I + I + I + I o T Reflected: IR A R S Absorbed: IA Transmitted: IT Incident: I0 Scattered: IS • Optical classification of materials: Transparent Translucent Opaque Single crystal Polycrystallin e & dense Adapted from Fig. 21.10, Callister 6e. (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.) Polycrystalline & porous Chapter 21 - 9 Monochromated light , v I0 , vI Reflected Transmitted Rayleigh scattering vscat v Raman scattering T = IT/I0 etc. I0 = IT + IA + IR T + A+R =1 Chapter 21 - 10 Atomic & Electronic Interactions Two Key Optical Phenomena… • Electronic Polarization: em wave includes fluctuating E-field E-field interacts with e- cloud around each atom perturbs or shifts e- cloud relative to nucleus: – some radiation energy may be absorbed – light waves retarded in vel. in medium…refraction • Electron Transitions: excitation e- from one occupied state to vacant higher energy state only specific values of E allowed all photon energy absorbed e- cannot remain in excited state forever…decays back to ground state re-emits photon(s) Chapter 21 - 11 Optical Properties of Metals: Absorption • Absorption of photons via electron transitions: Energy of electron Unfilled states E = h required! Io Planck’s Constant (6.63 x 10-34 J-s) Filled states Freq. of incident light Adapted from Fig. 21.4(a), Callister 7e. • Metals have a fine succession of vacant energy states • Near-surface electrons absorb visible light Chapter 21 - 12 Optical Properties of Metals: Absorption • All frequencies visible light absorbed: continuously available empty e- states above the Fermi level (highest occupied e- state) Ef total absorption in thin outer layer (< 0.1 m) metals are thus opaque to all em radiation at low end of f-spectrum most absorbed radiation re-emitted from surface, gen. w. same as incident…i.e. as reflected light Chapter 21 - 13 Optical Properties of Metals: Reflection • Electron transition re-emits a photon: Energy of electron IR Photon re-emitted from material surface unfilled states “conducting” electron E filled states Adapted from Fig. 21.4(b), Callister 7e. • Reflectivity = IR/Io, between 0.90 and 0.95, small % dissipated as heat! • Reflected light typ. same frequency ( ) as incident • Many metals appear reflective (silvery)! • For Cu, Au…some short photons not re-emitted 21 - 14 Chapter Optical Properties of Non-Metals • Insulators & Semiconductors: • Band structure…with band gaps • May be transparent or opaque to visible light • Thus, we need to also consider refraction and transmission… • Refraction: • “bending” of light…e.g. bent stick in water • related to electronic polarization…perturbation of ecloud relative to nucleus • magnitude = fn. (size of atoms/ions) • larger atoms/ions greater e- polarization, slower velocity in material, greater refraction (n) • n affects both optical path and influences % of incident light reflected at the surface Chapter 21 - 15 Refractive Index, n • Transmitted light distorts electron clouds no transmitted light + transmitted light + electron cloud distorts • Light is slower in a material vs. in vacuum n = refractive index c (velocity of light in vacuum) v (velocity of light in medium) - Adding large, heavy ions (e.g., lead can decrease the speed of light) - Light can be “bent” • Note: n = fn. ( ) Typical glasses ca. Polymers PbO (Litharge) Diamond 1.5 -1.7 1.3 -1.6 2.67 2.41 Selected values from Table 21.1, Callister 7e. Chapter 21 - 16 Some Values of n Origin is Electronic Polarization Chapter 21 - 17 Refraction - Color Separation • Each color ( ) refracted (deflected) by different amount (or angle) as it passes through medium (glass prism) Chapter 21 - 18 Reflectivity, R • Reflection: – Metals reflect almost all light – Copper & gold absorb in blue & green => gold color – Some light scattered at interface between media with different n, even if both media are transparent – Reflectivity R = fraction incident light reflected at interface… IR n2 n1 R= = I0 n2 + n1 2 nS 1 = nS + 1 light to surface • e.g.: Diamond 2.41 1 R= 2.41 + 1 2 vacuum/air into solid nair 1 2 = 0 . 17 17% of light is reflected Chapter 21 - 19 ...
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