Newton's Rings Apparatus Light from an extended monochromatic source is converted into a parallel beam of light by using a convex lens of short focal length and made to fall on an optically plane glass plate inclined at an angle of 45° to the vertical, where it gets reflected on to the Plano-convex lens. The wavelength of monochromatic light source is determined from the radii of the interference rings.Optical glass plate mounted at 45° (no adjustment required).Quick adjustment, good quality, well defined distinct fringes.