Figure a, shows two light rays that are initially in phase as they travel upward through a block of plastic, with wavelength 400 nm as measured in air. Light ray r1 exits directly into air. However, before light ray r2 exits into air, it travels through a liquid in a hollow cylinder within the plastic. Initially the height L liq of the liquid is 40.0?m, but then the liquid begins to evaporate. Let ? be the phase difference between rays r1 and r2 once they both exit into the air. Figure b shows ? versus the liquids height L liq until the liquid disappears, with ? given in terms of wavelength and the horizontal scale set by Ls = 40.00?m. What are?(a) The index of refraction of the plastic and(b) The index of refraction of the liquid?

60 20 Plastic Lig (um) (a) (b)