Level 0 bright stripes indicate that the optical path difference is 0, that is, the optical path is the same; R2 represents the optical path of the speed of light when there is no glass sheet (it should be R2 times 1, 1 represents the refractive index of light in air); R 1 represents another beam of optical path without glass sheet, and ne represents the optical path of light in glass sheet, minus the repeated part 1e, that is, (n- 1)e represents the redundant part without glass sheet. Every 1 level, the optical path difference increases λ.