Strong coupling occurs when light and matter strongly interact with each other. In order to reach the strong coupling regime, the strength of interaction between the two states, light and matter, has to be stronger than the dissipation rate. As a result of this, the two states will be hybridised to form part light part matter states, called the polaritons. This hybridisation is accompanied by a phenomenon called anti-crossing, where the two coupled states cannot degenerate with each other, which can be typically seen in their energy-momentum dispersion, where the dispersion of the original states avoid each other. Due to the modification in the energy of the system and the hybrid nature of the polaritons, it has been shown that properties of a system, such as the conductivity, the photophysical property, the excitation transport and even the chemical landscapes, can be altered. This thesis explores the origin of a couple of anti-crossings of confined electric fields at the energy in the absence of an electronic transition. The presence of surface charges and impedance matching can cause additional anti-crossing in the system and even shift the energy of the anti-crossing. These results show that the anti-crossing that occurs in systems with complicated optical constants has to be taken care of before associating it with the strong coupling of a confined electric field with molecular excitation. Although anti-crossing is a condition for a system to be in the strong coupling regime, it is not a unique characteristic of strong coupling. Strong coupling has been reported to be able to modify chemical reactions. However, this topic remains controversial, and there are a few studies that failed to observe such effect. In most of these experiments that claimed that the chemical landscape can be modified in the strong coupling regime, there is a lack of control studies. In the work of this thesis, I repeated one of these experiments with a more thorough methodology and have shown that, at least in this experiment, the strong coupling is not the main cause of such modification in the chemical reaction. This result shows that all the factors that could affect the chemical reaction in these polaritonic chemistry experiments must be thoroughly considered. The works presented in this thesis explore and tackle some of the strong coupling experiments with results that may cause misinterpretation and false analysis.Engineering and Physical Sciences Research Council (EPSRC)European Commission