Consider a Michelson interferometer in which the two arms are of equal length but the two plane mirrors are not perpendicular. One mirror can be adjusted to create a vertical air wedge. The angle between the two mirrors differs from 90° by a very small amount (less than few mrad). When the angle of the air wedge is 0, the two mirrors are perfectly perpendicular. The interference pattern is localized in the vicinity of the mirrors. It can be observed on a screen located at the conjugate plane of the mirrors using a converging lens. Assume a magnification equal to 1.