PVC film interference produced by the film. The film can be a transparent solid, a liquid or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the film to obtain the first light, the refracted light is reflected by the lower surface of the film, and then refracted by the upper surface to obtain the second light. The two lights are on the same side of the film and are separated by the same incident vibration. , Is coherent light, which belongs to sub-amplitude interference.
If the light source is an extended light source (surface light source), interference can only be observed in the specific overlapping area of the two coherent beams, so it is localized interference. For flat films with two surfaces parallel to each other, the interference fringes are localized at infinity, usually by means of a convergent lens to observe in the image square focal plane; for wedge-shaped films, the interference fringes are localized near the film.