Download Prova de Inglês - redemat

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.