Abstract: While Michelson interferometer and laser are used to measure the refractive index of glass pane，there is an abnormal phenomenon. When we put a glass pane into one side of the light path，we observe an abnormal phenomenon of equal inclination inference rings puff. In this letter，we analyze the phenomenon in terms of geometrical optics and make clear the relationship between the turning point of interference fringe，zero light path difference and the relative location of virtual light sources. We also perform comparative analysis with experimental data.

Key words: Michelson interferometer, turning point of interference fringe, refractive index, geometrical optics, virtual light source