  The optical properties of LiFeAs with $T_c \simeq$ 18 K have been determined
in the normal and superconducting states. The superposition of two Drude
components yields a good description of the low-frequency optical response in
the normal state. Below $T_c$, the optical conductivity reveals two isotropic
superconducting gaps with $\Delta_{1} \simeq 2.9$ $\pm$ 0.2 meV and $\Delta_{2}
\simeq 5.5$ $\pm$ 0.4 meV. A comparison between the superconducting-state
Mattis-Bardeen and the normal-state Drude components, in combination with a
spectral weight analysis, indicates that the spectral weight associated with a
band which has a very small scattering rate is fully transferred to the
superfluid weight upon the superconducting condensate. These observations
provide clear evidence for the coexistence of clean- and dirty-limit
superconductivity in LiFeAs.
