This lecture covers the interaction between light and matter in quantum systems, focusing on the temporal evolution of perturbed quantum systems and the formation of excitons. It explains the perturbation theory for quantum systems, the selection rules for electronic transitions, and the concept of excitonic coupling. The lecture also discusses the optical properties of molecules under light exposure, including absorption and emission spectra, Franck-Condon principle, and Davydov splitting. Additionally, it explores the exciton model in molecular spectroscopy, emphasizing the effects of exciton splitting on absorption bands and triplet state excitation.