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Unformatted text preview: 1.7 eV 3.4 eV 2.3 eV c o n d u c t io n b a n d ( e m p t y ) v a le n c e b a n d ( h a lf f il e d ) Silver X 1.7 eV 2.3 eV c o n d u c t io n b a n d ( e m p t y ) v a le n c e b a n d ( h a lf f il e d ) Gold ENGRI 111 Homework 2 Solutions, Sept. 25, 2002 1. Explain a. why silver appears silvery while b. gold appears golden. c. What color(s) of light would you expect to see when looking through a thin piece of gold? The colors exhibited by metals (and semiconductors) are due to the light that is absorbed and then re-emitted by the material. The electrons in the material are responsible for this absorption and re-emittance of light. The half-filled valence band of metals allows them to absorb a wide range of wavelengths in the visible spectrum. Silver is able to absorb all energies in the visible spectrum, and then re-emit them to give off a silvery white appearance. On the other hand, gold is only able to absorb light of low energies (red, orange, yellow) from the visible spectrum. Absorption of light of high energies (green, blue, indigo, violet) would promote the electrons of gold to the bandgap region, but this is a forbidden energy zone. The re-emittance of the lower energy light gives gold its golden appearance. Since gold does not absorb the higher energies of light, it must allow it to pass through (provided that the gold sheet is thin enough for transmission). The combination of these colors would give the light coming through a thin gold sample a purplish color. 2. Comment on the transparency (or lack thereof) in the infrared region for a. alumina, b. silver, and c. silicon....
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- Fall '07