Question

What is the nature of central fringe in Newtons rings experiment

Answer 1

A film is said to be thin when its thickness is about the order of one wavelength of visible light which is taken to be 550 nm. When light is incident on such a film, a small portion gets reflected from the upper surface and a major portion is transmitted into the film. Again a small part of the transmitted component is reflected back into the film by the lower surface and the rest of it emerges out of the film. These reflected beams reunite to produce interference. Also the transmitted beams too interfere. This type of interference that takes place in thin films is called interference by division of amplitude.

Answer 2

Therefore , at the center the geometrical path difference between the rays incident and reflected from glass plate is zero,but due to reflection a path difference of (lambda/2) is introduced. This path difference gives distructive interference at the center and hence center is dark.