  We investigated the temperature ($T=$ 15 $\sim $ 290 K) and the magnetic
field ($H=$ 0 $\sim $ 17 T) dependent optical conductivity spectra of a
charge/orbital ordered manganite, Nd$_{1/2}$Sr$_{1/2}$MnO$_3$. With variation
of $T$ and $H$, large spectral weight changes were observed up to 4.0 eV. These
spectral weight changes could be explained using the polaron picture.
Interestingly, our results suggested that some local ordered state might remain
above the charge ordering temperature, and that the charge/orbital melted state
at a high magnetic field (i.e. at $H=$ 17 T and $% T=$ 4.2 K) should be a three
dimensional ferromagnetic metal. We also investigated the first order phase
transition from the charge/orbital ordered state to ferromagnetic metallic
state using the $T$- and $H$% -dependent dielectric constants $\epsilon_1$. In
the charge/orbital ordered insulating state, $\epsilon_1$ was positive and
$d\epsilon_1/d\omega \approx 0$. With increasing $T$ and $H$, $\epsilon_1$ was
increased up to the insulator-metal phase boundaries. And then, $\epsilon_1$
abruptly changed into negative and $d\epsilon_1/d\omega >0$, which was
consistent with typical responses of a metal. Through the analysis of $%
\epsilon_1$ using an effective medium approximation, we found that the melting
of charge/orbital ordered states should occur through the percolation of
ferromagnetic metal domains.
