Advanced physics

Let Ap be the difference between the phase shift a Helium-Neon laser beam expe- riences on traversing a given length of vacuum and on traversing the same length of plasma. What is Ad when the laser beam passes through 10 cm of plasma having a density of n = 102? m?? How could this be used as a density diagnostic? The wavelength of the He-Ne laser radiation is 632.8 nm