1. The problem statement, all variables and given/known data One of the beams of an interferometer passes through a small glass container containing a cavity 1.30 cm deep. When a gas is allowed to slowly fill thr container, a total of 236 dark fringes are counted to move past a reference line. The light used has a wavelength of 610 nm. Calculate the index of refraction of the gas, assuming the interferometer is in a vacuum 2. Relevant equations extra distance = m*lambda/n = twice the depth d=vt ?? 3. The attempt at a solution The first dark spot occurs when the extra distance is half the new lambda. This means the 236th dark spot occurs when m = 235.5 I attempted to solve for n by equating this to twice the depth, but got something way lower than 1. I must be missing something big. Does the glass container outside play a role?