Question: One of the beams of an interferometer, as seen in the figure below, passes through a small glass container containing a cavity D = 1.40 cm deep. When a gas is allowed to slowly fill the container, a total of 230 dark fringes are counted to move past a reference line. The light used has a wavelength of 600 nm. Calculate the index of refraction of the gas, assuming that the interferometer is in vacuum. I'm not even sure where to start with this question.. any help?