What are the Newton’s rings and how are they formed?

Newton’s rings and its formation : 

When a Plano-convex lens of long focal length is placed on a plane glass plate, a thin film of air is enclosed between the lower surface of the lens and the upper surface of the plate. The thickness of the air film is very small at the point of contact and gradually increases from the center outwards. 

The fringes produced with monochromatic light are circular. The fringes are concentric circles, uniform in thickness and with the point of contact as the center. When viewed with white light, the fringes are colored. With monochromatic light, bright and dark circular fringes are produced in the air film. Since the phenomenon was first examined in detail by Newton, the rings are termed by Newton’s rings.