What is the fringe width of NEWTON RING?

The fringe width of Newton rings is the distance between two consecutive bright or dark fringes observed when a plano-convex lens is placed on a flat glass plate. It is given by the formula [ w = \lambda \cdot R / (D - R) ], where ( \lambda ) is the wavelength of light, ( R ) is the radius of curvature of the lens, and ( D ) is the diameter of the bright ring.