Rich colors decorating oil slicks on water are interference fringes. These fringes arise from the constructive and destructive interference of the multicomponents of light beams generated from partial reflection of the incident light beam at the first and second surfaces of a transparent sample having plane parallel surfaces. Fringes are reflection (or transmission) maxima and minima and appear when the wavelength of the incident light, the angle of incidence, or film thickness is changed. Interference fringes are very useful since advantage can be taken of them in many applications, including measurement of film thickness and refractive index n1; constructions of interference filters, cold mirrors, and high Q optical cavities; and modulation of light, to name a few.
N. J. Harrick, "Transmission Spectra without Interference Fringes," Appl. Spectrosc. 31, 548-549 (1977)