  In this communication we present a detailed study of the effect of chemical
disorder on the optical response of Ni$_{1-x}$Pt$_x$ (0.1$\leq$ x $\leq$0.75)
and Ni$_{3(1-x)/3}$Pt$_x$ (0.1$\leq$ x $\leq$0.3). We shall propose a formalism
which will combine a Kubo-Greenwood approach with a DFT based tight-binding
linear muffin-tin orbitals (TB-LMTO) basis and augmented space recursion (ASR)
technique to explicitly incorporate the effect of disorder. We show that
chemical disorder has a large impact on optical response of Ni-Pt systems. In
ordered Ni-Pt alloys, the optical conductivity peaks are sharp. But as we
switch on chemical disorder, the UV peak becomes broadened and its position as
a function of composition and disorder carries the signature of a phase
transition from NiPt to Ni$_3$Pt with decreasing Pt concentration.
Quantitatively this agrees well with Massalski's Ni-Pt phase diagram
\cite{massal}. Both ordered NiPt and Ni$_3$Pt have an optical conductivity
transition at 4.12 eV. But disordered NiPt has an optical conductivity
transition at 3.93 eV. If we decrease the Pt content, it results a chemical
phase transition from NiPt to Ni$_3$Pt and shifts the peak position by 1.67 eV
to the ultraviolet range at 5.6 eV. There is a significant broadening of UV
peak with increasing Pt content due to enhancement of 3d(Ni)-5d(Pt) bonding.
Chemical disorder enhances the optical response of NiPt alloys nearly one order
of magnitude. Our study also shows the fragile magnetic effect on optical
response of disordered Ni$_{1-x}$Pt$_x$ (0.4$<$ x $<$0.6) binary alloys. Our
theoretical predictions agree more than reasonably well with both earlier
experimental as well as theoretical investigations.
