The interference of light has been analyzed for a film structure by considering that a spatial separation exists for the two neighboring light beams to be interfered in the space. There is a significant difference between the situations of the interference with or without consideration of the spatial effect, especially around the region where the phase delay \delta =\pi and 2π by taking the example of the one-layered {\text{SiO}}_{\text{2}}/\text{Si} structure. It is reasonable to extract the optical parameters by neglecting the spatial effect only for the thinner film with a thickness much smaller than a wavelength, which satisfies the condition that \delta <\pi ; otherwise, the film equation used for the periodic or nonperiodic structures will be modified by including the spatial effect in the data analysis and applications.