  We performed optical spectroscopy measurement on single crystals of
BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based
superconductors. Both are found to be quite metallic with fairly large plasma
frequencies at high temperature. Upon entering the spin-density-wave (SDW)
state, formation of partial energy gaps was clearly observed with the presence
of surprisingly two different energy scales. A large part of the Drude
component was removed by the gapping of Fermi surfaces (FS). Meanwhile, the
carrier scattering rate was even more dramatically reduced. We elaborate that
the SDW instability is more likely to be driven by the FS nesting of itinerant
electrons rather than a local-exchange mechanism.
