The different lengths that they are measuring after their fix are most likely because the transparent plastics (n_p ≈ 1.5) has a different refractive index than

The different lengths that they are measuring after their fix are most likely because the transparent plastics (n_p ≈ 1.5) has a different refractive index than air (n_air = 1). That is the optical path length is no longer the same. https://en.m.wikipedia.org/wiki/Optical_path_length https://en.m.wikipedia.org/wiki/Optical_path_length