Question

How do circular fringes originate in Michelsons interferometer
​

Answer 1

Explanation:

have never done the experiment so as far as I understand, if the mirrors M1M1 and M2M2 are perpendicular to each other the observer at OO will see a circular fringe pattern. If additionally M1M1 and M2M2 are equidistant from the beam-splitter P1P1 (Δd=0Δd=0), then the observer does not see any ring (just a single dark fringe).