The electrodynamic response of the organic spin-liquid candidate $kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ has been measured in an extremely wide energy range ($10^{-13}$ to 2 eV) as a function of temperature (5 to 300 K). Below the Mott gap, excitations
from the un-gapped spinon continuum cause a considerable contribution to the infrared conductivity, as suggested by the U(1) gauge theory. At THz frequencies we can identify a power-law behavior $sigma(omega) propto omega^{beta}$ with two distinct exponents $beta$ that change from 0.9 to 1.3 at low temperatures. The corresponding crossover scales with temperature: $hbaromega_c approx k_B T$. The observed exponents differ by more than a factor of 2 from the theoretically predicted ones. The findings are compared with those obtained on Herbertsmithites.
The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin film were measured and the optical conductivity and permittivity evaluated over a wide frequency range. The open
ing of the superconducting gap $2Delta_0 = 3.7$ meV below $T_capprox 20$ K is {em directly} observed by a completely vanishing optical conductivity. The temperature and frequency dependent electrodynamic properties of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ in the superconducting state agree well with the BCS predictions with no nodes in the order parameter. The spectral weight of the condensate $1.94times 10^7 {rm cm}^{-2}$ corresponds to a London penetration depth $lambda_L=3600$ AA.
