A systematic study on the optical properties of thermally heated CR-39 polymer samples has been carried out. The recorded UV-visible transmission and reflection spectra have been used to determine the absorption coefficient for these samples. From these spectra, the optical band gap and refractive index of the annealed samples have been determined. The reduction in optical band gap and enhancement in refractive index with a rise in annealing temperature have been observed, with the possibility to tune these optical properties as per requirements. Finally, an attempt has been made to explain the changes in these optical parameters with the induced structural changes in this polymer, revealed through Raman spectroscopy, as a result of heating.