SKEDSOFT
Central spot ‘O', is dark although the film thickness is zero there, This is because of the phase difference of π(equivalent path difference = λ/2) introduced in the ray reflected by flat glass plate.
The radius of the nth 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 nth 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 λ the diameter 'D' (=2x) of every 5th ring is measured, while the space between the plano convex lens and flat surface has only air film 
. In this case, for bright rings .
and 
thus 
One can also plot 
Vs n and fit the experimental data to a straight line ( Y = MX C ) using least square fitting method. From the intercept ‘C' the wavelength of light can be determined.
To determine the refractive index of liquid using newton's ring method, first the diameter of different order bright rings are measured using air film. Then liquid is inserted between the glass surface and diameter is again measured. The ratio
gives the refractive index of liquid.
