Example: Two 20cm glass tubes are placed in the two arms of a Michelsons interferometer respectively. One is vacuum and the other is filled up with Ar gas. Considering the light from extended source S(2=5460A) falls on the interferometer. If Ar gas is drawn off. a shift of 205 fringes occurs. Find the refractive index of Ar gas Solve M 2(n-1)=△ n-1=△2/2lS GG 2 205×5.46×10 2×0.20 =0.28×10 E n=1.00028Example：Two 20cm glass tubes are placed in the two arms of a Michelson’s interferometer respectively. One is vacuum and the other is filled up with Ar gas. Considering the light from extended source S（=5460A）falls on the interferometer. If Ar gas is drawn off , a shift of 205 fringes occurs. Find the refractive index of Ar gas. E S M1 M2 G1 G2 L 2(n −1)l = k n−1= k / 2l 2 0.20 205 5.46 10 7    = − Solve： 3 0.28 10− =  n =1.00028 l l