The conventional wisdom is that the high-temperature cuprate superconductors are in the clean limit due to their short coherence length. In the clean limit the opening of the superconducting energy gap should be difficult to observe in the optical properties; however, in the cuprates the gapping of the spectrum of excitations with the onset of superconductivity usually has a dramatic optical signature that is suggestive of a system in the dirty limit. To provide guidance, the optical properties of the multiband iron-based superconductor FeTe0.55Se0.45 (Tc~14 K) have been examined. The iron-based material may be described by two bands; one band is coherent, while the other is nearly incoherent, placing this material simultaneously in both the clean and dirty limits . While the optical properties of the cuprates are described by a single band, both it and the superconducting gap are momentum dependent. The similarity of these two classes of materials suggests that the cuprates may also behave as if they are in both the clean and dirty limit at the same time, a view that is supported by the fact that the cuprates and the iron-based superconductors both fall on the same universal scaling line .
 C. C. Homes et al., Phys. Rev. B 91, 144503 (2015).
 C. C. Homes et al., Nature 430, 539 (2004).