Newton rings are formed by interference of thin films.When a plano convex lens of long focal length is placed over an optically plane glass plate, a thin air film with varying thickness is enclosed between them.The thickness of air film is zero at the point of contact and gradually increases outward from the point of contact.When the air film is illuminated by a monochromatic light normally,alternate bright and dark concentric circular rings are formed with dark spot at the centre.These rings are known as newton's rings.