With the above analysis, we have theoretically justified why the VSC-modified chemical kinetics only occurs at the normal incidence when 𝜔 c = 𝜔 0 for a 2D FP cavity, which agrees with experimental observations . This is because even though there is no singularity in 𝑔 2D (𝜔), the cavity loss along the 𝑘 ‖ direction forces the cavity spectra function 𝒜(𝜔) to peak up at 𝜔 = 𝜔 c , which forces the normal incidence condition. The condition for observing Rabi splitting (see Eq. 9), on the other hand, is 𝜔 k = 𝜔 c √ 1 + tan 2 𝜃 = 𝜔 0 for any 𝜃 ≥ 0. Although the modes with 𝜃 > 0 barely contribute to 𝑘 VSC , the mode density is finite (see Fig. ) and for 𝜔 0 > 𝜔 c there will always be a mode available that satisfies 𝜔 k = 𝜔 0 , generating Rabi splitting at 𝜃 > 0. As such, the theory provides a clear understanding of the fundamental difference between the condition for forming the Rabi splitting and that of the VSC resonance modification of the rate constant. This explains the experimentally observed resonance phenomena that occur only at 𝜔 c = 𝜔 0 at the normal incident angle when 𝑘 ‖ = 0 (or 𝜃 = 0), but not at a finite angle of 𝜃 even though the resonance condition for generating Rabi splitting is fulfilled.