Although the photon absorption losses were lower than expected, they were most pronounced in cases where gas bubbles nearly spanned the full axial length of the reactor. The results imply that a fraction of the photons which are initially transmitted after interacting with large gas bubbles find their way back into the reaction mixture. Total internal reflection can occur at the interface between the gas bubble and the reaction mixture, as well as between the reactor material (PFA or quartz) and the surrounding air. This allows the reactor material to function as a waveguide, preventing photon losses and enabling those photons to be absorbed upon re-entering the reaction mixture. The transition from large bubbles to smaller, spherical gas bubbles, as demonstrated in this study, led to increased photon absorption. Light rays scatter and change direction at these complex interfaces, and when combined with total internal reflection, this can extend the effective optical path length. The results obtained apply to the specific experimental conditions and setup used, which include factors such as the light source, solvent, choice of (inert) gas, reactor material, and setup configuration. Nevertheless, these findings are likely to be relevant to other photoreactor systems and chemistries involving non-absorbing gases, similar reactor designs, bubble behavior, and comparable light source characteristics.