Interference of two diverging beams reflected from a plane parallel plate: Stationary fringes with characteristic stable period

We investigated interference fringes due to superposition of diverging light waves reflected from the two sides of a plane parallel plate. A non-localized fringe pattern of high contrast was obtained as a function of incident angle when we used a coherent diverging beam. We found that the fringe density increased to a certain angle and then decreased thereafter, against the common belief that the fringe density increases monotonically with the angle of incidence. Because the fringe density is maximum at this angle and does not change rapidly in the vicinity, we could employ Fourier analysis to estimate interferometric parameters, such as thickness, refractive index, and wavelength, that determine the characteristic stationary fringe density.