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.