One leg of a Michelson interferometer contains an evacuated cylinder of length L, having glass plates on each end. A gas is slowly leaked into the cylinder until a pressure of 1 atm is reached. If N bright fringes pass on the screen when light of wavelength λ is used, provide an expression for the index of refraction of the gas?
Hint: The fringe shifts occur because the wavelength of the light changes inside the gas-filled tube.