It is a meaningful exercise that discussing the patterns of interference fringes in case introducing polarizers into the Young's double slit experiment while different combinations. However, “no fixed phase difference,” the commonly seen answer, does not fully understand the essence of the problem. By utilizing Malus's Law and the principle of wave superposition, this paper calculates the resulting contrast after superposition. Quantitative calculations demonstrate that when three polarizers are introduced in Young's double-slit interference experiment, the physical reason for the absence of interference fringes is the existence of a projection phase difference. Moreover, it is emphasized that in this context, the projection phase difference is the determining factor for the presence or absence of interference fringes. The article provides a clear and intuitive physical image, suitable for classroom teaching, displaying the role of the projected phase difference and aiding students in gaining a deeper understanding and application of concepts related to light emission mechanisms, interference, polarization, and the interference of polarized light.