Abstract: The basic analytic method and phenomena of electromagnetic interference is explained through the discussions of Young＇s experiment of polarized light within the framework of fundamental optics based on the superposition of vectorial waves, including the interference of two linearly polarized beams and the case of one linear beam passing through a pair of wave plates of arbitrary thickness. It shows that generally the spatial mod- ulation of polarization will occur at the observation plane with or without intensity modulation. These analyses indicate the important role of vectorial properties of light waves in interference and may uncover some physical effects of interference hidden when only the intensity fringes are concerned.

Key words: Young＇ s experiment, electromagnetic interference, superposition of vectorial waves, polarization, contrast