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L34 - SUMMARY FROM LAST CLASS Optical properties of...

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Dr. P. Lucas U of A MSE 110 Optical properties of materials SUMMARY FROM LAST CLASS The optical window of a material is the range of wavelength where the absorption coefficient α is close to zero. The optical window of a transparent solid is limited at short wavelength by the electronic transitions (absorption of a photon to promote an electron into the conduction band). The cut-off wavelength is therefore defined by the bandgap as λ c =hc/E g . At long wavelength the optical window is limited by the atomic vibrations induced by the oscillating electric field of the light. The vibrational frequency decrease with the mass of the atoms. The addition of impurities in the material (typically transition metals) generates absorption peaks within the transparency window and produces colors. The absorption coefficient α increases with the concentration of impurities. l α = e o I I The fraction of absorbed light is a function of the path length through the sample and the absorption coefficient α of the material.
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Dr. P. Lucas U of A MSE 110 Refractive index of solids OPTICAL DEVICES The refractive index describes the ability of the electron density in the solid to distort in response to the oscillating electric field of an electromagnetic wave. The refractive index is therefore related to the dielectric constant according to . ε = n This interaction reduces the velocity of the wave as it proceed through the solid and the refractive index also quantify that decrease in velocity according to .
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