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.
7 Transmitted light distorts electron clouds. Result 1: Light is slower in a material vs vacuum. Index of refraction (n) = speed of light in a vacuum speed of light in a material Material Lead glass Silica glass Soda-lime glass Quartz Plexiglas Polypropylene n 2.1 1.46 1.51 1.55 1.49 --Adding large, heavy ions (e.g., lead can decrease the speed of light. --Light can be "bent" Result 2: Intensity of transmitted light decreases with distance traveled (thick pieces less transparent!) Selected values from Table 21.1, Callister 6e. TRANSMITTED LIGHT: REFRACTION
c19f05 Figure 19.5 Non Metallic Materials—Materials with a bandgap an electron is excited across the band gap leaving behind a hole in the valence band. absorbed photon energy: E = Eg Emission of a photon of light by a direct electron transition across the band gap.
c19f06 Figure 19.6 Photon absorption via a valence band-conduction band electron excitation for a material that has an impurity level that lies within the band gap Emission of two photons involving electron decay first into an impurity state, and finally into the ground state Generation of both a phonon and a photon as an excited electron falls first into an impurity level and finally back to its ground state
5 Absorption by electron transition occurs if h > E gap If E gap < 1.8eV, full absorption; color is black (Si, GaAs) If E gap > 3.1eV, no absorption; colorless (diamond) If E gap in between, partial absorption; material has a color. Adapted from Fig. 21.5(a), Callister 6e. SELECTED ABSORPTION: NONMETALS incident photon energy h
c19f07 Figure 19.7 The transmission of light through a transparent medium for which there is reflection at front and back faces, as well as absorption within the medium.
Color determined by sum of frequencies of --transmitted light, --re-emitted light from electron transitions. Ex: Cadmium Sulfide (CdS) -- E gap = 2.4eV, -- absorbs higher energy visible light (blue, violet), -- Red/yellow/orange is transmitted and gives it color. Ex: Ruby = Sapphire (Al 2 O 3 ) + (0.5 to 2) at% Cr 2 O 3 -- Sapphire is colorless (i.e., E gap > 3.1eV) -- adding Cr 2 O 3 : alters the band gap blue light is absorbed yellow/green is absorbed red is transmitted Result: Ruby is deep red in color. Adapted from Fig. 21.9, Callister 6e. (Fig. 21.9 adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.) COLOR OF NONMETALS
the light transmittance of three aluminum oxide specimens. From left to right: single- crystal material (sapphire), which is transparent; a polycrystalline and fully dense (nonporous) material, which is translucent; and a polycrystalline material that contains approximately 5% porosity, which is opaque. 19.10 OPACITY AND TRANSLUCENCY IN INSULATORS
(a)The chromium ions before excitation (b)Electrons in some chromium atoms are excited into higher energy states by the xenon light flash. (c)Emission from metastable electron states is initiated or stimulated by photons that are spontaneously emitted. (d)Upon reflection from the silvered ends, the photons continue to stimulate emissions as they traverse the rod length. (e)The coherent and intense beam is finally emitted through the partially silvered end. The stimulated emission and lightamplification for a ruby laser.
12 When light (radiation) shines on a material, it may be: --reflected, absorbed and/or transmitted. Optical classification: --transparent, translucent, opaque Metals: --fine succession of energy states causes absorption and reflection. Non-Metals: --may have full (E gap 3.1eV), or partial absorption (1.8eV < E gap = 3.1eV). --color is determined by light wavelengths that are transmitted or re-emitted from electron transitions. --color may be changed by adding impurities which change the band gap magnitude (e.g., Ruby) Refraction: --speed of transmitted light varies among materials. SUMMARY