Engineering Physics

14.5 Newton’s rings

As shown in Fig. 14.5, a plano convex lens (L) having long focal length, f (≈100 cm) is placed with its convex surface on a plane glass plate (G). A gradually increasing thickness of air film will be formed between plane glass plate and convex surface of plano convex lens. The thickness of air film will be zero at the point of contact and symmetrically increases as we go radially from the point of contact. A monochromatic light of wave length ‘λ’ is allowed to fall normally on the lens with the help of a glass plate ‘P’ kept at 45° to the incident monochromatic beam. A part of the incident light rays are reflected up at the convex surface of the lens and the remaining light is transmitted through the air film. Again a part ...