In Opacity and Translucency Visible light would normally transmit completely, but impurities can yield color Most ceramics are insulators ( Large Eg Why are they usually opaque and white? E>3.1ev Why Are Most Ceramics Opaque? Why are Metals Shiny? d transmittance gRain boundarie Metals e=oev nisotropic n(different grains at different All light with n above X-ray wavelengths absorbed by Light is reemitted with exact energy of absorption as aboth refraction and reflection occur electrons fall back into lowest state. Metals appear tWo phase materials cause scattering reflective as the light we see is reemitted. When there is a difference in n, the greater the difference, the greater the scattering e.g. porosity is very effective in scattering light When particles are about the averag wavelength of visible light, all light wavelengths absorption emIssion are refracted, yielding a whitish, op Applications of Optical Phenomena Applications of Optical Phenomena o Luminescence o Materials which are capable of absorbing energy la ser n (light, heat, electron beam) then re"emitting a device that utilizes the natural oscillations visible light o Time of delay between absorption of energy and of atoms or molecules between energy levels for generating coherent electromagnetic s fluorescence (less than one second) radiation usually in the ultraviolet, visible, or e phosphorescence(greater than a Television O LEDs light)a(mplification by)s(timulated)e(mission of radiation3 Impurities ß Visible light would normally transmit completely, but impurities can yield color Eg > 3.1 eV Opacity and Translucency ß Most ceramics are insulators (Large Eg ) ¾ Why are they usually opaque and white? Why Are Most Ceramics Opaque? ¾¾ Internal reflectance and transmittance Grain boundaries anisotropic n (different grains at different orientations) both refraction and reflection occur Two phase materials cause scattering when there is a difference in n, the greater the difference, the greater the scattering e.g. porosity is very effective in scattering light When particles are about the average wavelength of visible light, all light wavelengths are refracted, yielding a whitish, opaque color. Why are Metals Shiny? Metals Eg = 0 eV All light with l above X-ray wavelengths absorbed by continuous unoccupied states above Ef . Light is reemitted with exact energy of absorption as electrons fall back into lowest state. Metals appear reflective as the light we see is reemitted. absorption emission Ef Applications of Optical Phenomena Luminescence Materials which are capable of absorbing energy (light, heat, electron beam) then re-emitting visible light Time of delay between absorption of energy and reemission varies fluorescence (less than one second) phosphorescence (greater than one second) Television! LED’s la·ser n. a device that utilizes the natural oscillations of atoms or molecules between energy levels for generating coherent electromagnetic radiation usually in the ultraviolet, visible, or infrared regions of the spectrum l(ight) a(mplification by) s(timulated) e(mission of) r(adiation) Applications of Optical Phenomena