  Y. M. Dai et al. in their recent work arXiv:1106.4430 presented a
reflectivity study of the in-plance optical conductivity of a
$\textrm{Ba}_{0.6}\textrm{K}_{0.4}\textrm{Fe}_2\textrm{As}_2$ (BKFA)
superconductor with $T_{\textrm{c}}\approx39K$. The single crystals used in
this study are of high quality and the measurements are rather accurate. The
authors analyzed the optical conductivity of BKFA in the framework of the
Mattis-Bardeen theory and the BCS theory of superconductivity. This analysis,
however, generates a series of severe contradictions with a large body of
experimental data obtained with multiple probes as well as misuses both the
Mattis-Bardeen and the BCS theories of superconductivity beyond their
approximation regions.
