A michelson interferometer is used with a helium-neon laser of wavelength 628.8 nm. A glass-walled chamber of interior length 1.005 cm is placed in one of the beams and the position of the mirror is adjusted until bright light is observed at the detector. The glass chamber is then evacuated. During the evacuation, the viewed light changes from bright to dark and back to bright nine times. What is the refractive index of air?