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UFOP - CETEC - UEMG Rede Temática em Engenharia de Materiais Ouro Preto, 18 de fevereiro de 2008. UFOP - CETEC - UEMG Exame de Seleção para o ingresso no Mestrado em Engenharia de Materiais Primeiro Semestre de 2008 Prova de Inglês Instruções ao candidato: 1. Leia o texto abaixo e redija, em português, separadamente e na mesma ordem, cada um dos parágrafos presentes no original em inglês. Todas as informações importantes devem estar presentes na tradução e será considerada a qualidade do texto em português. 2. O exame terá uma duração de 1:30h (uma hora e trinta minutos). Adapted from "Fundamentals of Materials Science and Engineering - W. D. Callister" The optical behavior of a solid material is a function of its interactions with electromagnetic radiation having wavelengths within the visible region of the spectrum. Possible interactive phenomena include refraction, reflection, absorption, and transmission of incident light. Metals appear opaque as a result of the absorption and then reemission of light radiation within a thin outer surface layer. Absorption occurs via the excitation of electrons from occupied energy states to unoccupied ones above the Fermi energy level. Reemission takes place by decay electron transitions in the reverse direction. The perceived color of a metal is determined by the spectral composition of the reflected light. Nonmetallic materials are either intrinsically transparent or opaque. Opacity results in relatively narrow band gap materials as a result of absorption whereby a photon’s energy is sufficient to promote valence band-conduction band electron transitions. Transparent nonmetals have band gaps greater than about 3 eV. Wide-band gap insulators that contain impurities may appear colored, and the color depends on the distribution of wavelength ranges in the transmitted beam. Light radiation experiences refraction in transparent materials; that is, its velocity is retarded and the light beam is bent at the interface. Index of refraction is the ratio of the velocity of light in a vacuum to that in the particular medium. The phenomenon of refraction is a consequence of electronic polarization of the atoms or ions, which is induced by the electric field component of the light wave. When light passes from one transparent medium to another having a different index of refraction, some of it is reflected at the interface. The degree of the reflectance depends on the indices of refraction of both media, as well as the angle of incidence. Normally transparent materials may be made translucent or even opaque if the incident light beam experiences interior reflection and/or refraction. Translucency and opacity as a result of internal scattering may occur in polycrystalline materials that have an anisotropic index of refraction, in two-phase materials, in materials containing small pores, and in highly crystalline polymers. Three other important optical phenomena should be discussed; luminescence, photoconductivity, and lasers. With luminescence, energy is a absorbed as a consequence of electron excitations, which is reemitted as visible light. Photoconductivity consists of the enhancement of the electrical conductivity of some semiconductors by the generation of additional free electrons and electron-holes when photons collide with their electrospheres. Lasers consist of coherent and high-intensity light beams that are produced by the light amplification by stimulated emission of radiation.