A cadmium sulfide crystal's measured absorption

PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Semiconductor physics and devices · Dielectric and optical properties of solids

Condensed Matter Physics, Ch Electrons in a periodic potential: Bloch's theorem and the opening of a gap

Problem

A cadmium sulfide crystal's measured absorption, in arbitrary units, is given at six wavelengths. Data: at λ=600 nm\lambda=600\,\mathrm{nm} the absorption is 0.020.02; at 550550 it is 0.050.05; at 520520 it is 0.300.30; at 512512 it is 0.550.55; at 500500 it is 0.900.90; at 480480 it is 0.980.98. The absorption rises sharply through 512 nm512\,\mathrm{nm}, which is taken as the edge. ((a)) Find the band gap in electronvolts. ((b)) Would light of wavelength 450 nm450\,\mathrm{nm} (blue) pass through a thin wafer of this crystal, or be absorbed? ((c)) Would light of wavelength 650 nm650\,\mathrm{nm} (red) pass through, or be absorbed?

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