In this thesis the optical properties of $AgGaSe_2$;Co is studied . Photoacoustic spectroscopy (PAS),photoreflectance (PR), reflectance and transmittance spectra were carried out in order to measure the energy gap of $AgGaSe_2$;Co , the transition peaks at $E_1$,$E_2$ and $E_3$ of $Co^{2+}$ ion in $AgGaSe_2$ crystal and spin-orbit splittings at $E_3$ transition. The Eg were 1.78 eV for PAS, 1.82 eV for PR, and 1.77 eV for reflectance spectroscopy. $E_2$ transition was 0.75 eV and $E_3$ transition was 1.53 eV. Racah parameter B and crystal field theory parameter Dq were 348$cm^{-1}$, 524$cm^{-1}$ respectively. Three spin-orbit splittings of $E_3$ peaks were obserbed at 1.68 eV, 1.57 eV and 1.50 eV , from which the spin-orbit splitting parameter A was determined to be 0.045 eV.
