The wavelength of the light source must be similar to the thickness of the film through which it travels for interference fringes to be noticeable.
Work Step by Step
This question is best answered by first considering an “air wedge,” as in Figure 24–33. The fringes are equally spaced because as you look at points farther from the contact point on the left, the path differences between the rays change linearly. On the other hand, with a curved piece of glass, as you look at points farther away from the contact point at the center, the path differences change more and more rapidly (Figure 24-31). The higher order interference rings occur in a smaller interval, i.e., they are more tightly spaced.