Radiation-matter strong coupling leading to hybrid states and thereby modified spectroscopic and chemical properties has garnered significant experimental and theoretical interest in recent years. Within Fabry-Pérot cavities, strong coupling is typically achieved by collective interactions of many chromophore molecules with cavity modes whereas for plasmonic cavities, the cavity field-matter coupling can be large enough to manifest strong coupling involving even a single molecule. This dissertation develops theoretical understanding on important aspects and implications of light-matter interactions in both types of cavities especially in the context of modified optical properties using different semiclassical techniques. The first part concentrates on plasmonic cavities, in particular we investigate the behavior of coupling and lifetime of single molecules at plasmonic interfaces and how they depend on geometric and material properties of the nanostructures. We also extend our treatment to certain infrared plasmonic cavities to extract parameters relevant in vibrational strong coupling experiments. The second part of the dissertation focuses on vibronic strong coupling of many molecules in Fabry-Pérot cavities and thereby the effect of the cavity field on the nuclear dynamics, especially the Franck-Condon envelope. Our results provide insights into the effect of collective molecular response on different optical processes involved in such experiments.