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The absorption coefficient for excitons is calculated in the effective-mass approximation. The coefficient is shown to be decreased over what one would expect for a collection of free atoms by the factor ε^-1(a₀aB)³, where ε is the dielectric constant and a₀aB is the ratio of the lattice constant to the exciton radius. This result seems not inconsistent with the identification of the observed absorption lines in Cu₂O, CdS, HgI₂, PbI₂, and CdI₂ as exciton lines.
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