In this thesis the optical properties of $AgInSe_2$ and $AgInSe_2$:Co polycrystals were studied. The crystals were prepared by the melt growth method. First we investigated the lattice constants of these materials. The Co impurity did not change the lattice constants of $AgInSe_2$. But optical absorption spectra showed the Co impurity effect. The optical band gap was reduced by the addition of Co impurity(1.18 eV $\to$ 1.15 eV). The absorption spectra of $Co^{2+}$ ion showed the spin-orbit splitting of $^4A_2 \to ^4T_1(^4F)$ transitions of $4808 cm^{-1},\; 5430 cm^{-1},\; 6430 cm^{-1}$. And we could determine the crystal field splitting parameter Dq($324 cm^{-1}$) and spin-orbit coupling parameter $\lambda(271 cm^{-1})$.