(dark ring)

                   (bright ring)

 

Central spot ‘O', is dark although the film thickness is zero there, This is because of the phase difference of (equivalent path difference = ) introduced in the ray reflected by flat glass plate.

The radius of the n^th dark or bright ring can be measured by measuring the diameter of the ring. In an experimental set up the cross wire of eye piece is focused on the n^th order ring on both side the center spot respectively by moving the micrometer screw and noting down the corresponding reading on micrometer scale. For an accurate determination of the diameter 'D' (=2x) of every 5^th ring is measured, while the space between the plano convex lens and flat surface has only air film . In this case, for bright rings .