15.7 Newton's Rings
Fig 15.14 shows the convex surface of a lens in contact with a plane glass plate. A thin film of air is formed between the two surfaces. When we view the setup with monochromatic light, you see circular interference fringes (Fig. 15.15). These were studied by Newton and are called Newton's rings.
(Fig. 15.14: convex lens in contact with glass plate)
(Fig. 15.15: Circular interference fringes)
Newton's rings are formed when a plano-convex lens is placed on a flat glass plate, a thin air film forms between them. Light from a monochromatic source (like a sodium lamp) reflects off both the top and bottom surfaces of this air film. The reflected rays interfere-constructively (bright rings) or destructively (dark rings)-depending on the path difference and phase shift between them. Newton's Rings can be used to calculate the wavelength of monochromatic light, helps determine the radius of curvature of lenses and used in precision optics to detect surface irregularities