which is the cavity frequency we introduced in the previous discussions (Eq. 4). Experimentally, it is observed that only when ω c = ω 0 (known as the normal incidence condition) will there be VSC effects . For a red-detuned cavity (ω c < ω 0 ), there are still a finite number of modes (with a finite value of k ∥ ), such that ω k = ω 0 . This is referred to as the oblique incidence, but there is no observed VSC effect even though polariton states are formed . As mentioned in Ref. , "Tuning the cavity so that VSC occurs at normal incidence is essential to observe the modification of chemical properties. In this condition, the system is at the minimum energy in the polaritonic state ". Despite recent theoretical progress , there is no accepted theoretical explanation for VSC effect only observed at the normal incidence, although intuitively, the group velocity ∂ω k /∂k ∥ = 0 at k ∥ = 0 and make that point special, as hinted by Ebbesen and co-workers . Our recent work suggests that for the analytic expression k VSC (Eq. 2), it is possible to explain such a normal incidence effect when considering many cavity modes . In this work, we theoretically explore such normal incidence conditions for the new analytic expression kVSC (Eq. 17) under the lossless regime.