Here, I will show what happens if one modifies the inner interfaces. In contrast to figure 3, only secondary fringes change their wavelengths as the refractive index in the gap varies. This is because primary fringes do not arise from reflections at the inner interfaces. As a consequence, and, against common practice, the shape of primary fringes must be used with caution to determine the chromatic order of a primary fringe. Due to their nature, secondary fringes probe the surface topography.
Secondary fringes curve according to variations in the thicknes of the corresponding surface layer.
Looking at the shape of secondary fringes to gain topographic information is a very sensitive method. The mirror reflectivity in this picture was tuned to match the reflectivity of the inner interfaces. This system is symmetric, therefore, both secondary fringe patterns are identical. Figure 6 shows the growth of the same bulge on one surface only. The interferometer is now increasingly asymmetric and one can observe 'beating' effects between the two secondary patterns.