A gas-filled cell of length 5 cm is inserted in one arm of a Michelson interferometer. The interferometer is illuminated by light of wavelength 500 nanometres. As the gas is evacuated from the cell, 40 fringes cross a point in the field of view. The refractive index of this gas is most nearly?
1
1.02
2
1.002
3
1.0002
4
1.00002