  In an earlier work (Shankar kumar Jha, A Vyas, O S K S Sastri, Rajkumar Jain
& K S Umesh, 'Determination of wavelength of laser light using Modified
Newton's rings setup', Physics Education, vol. 22, no.3, 195-202(2005))
reported by our group, a version of Newton's rings experiment called Modified
Newton's rings was proposed. The present work is an extension of this work.
Here, a general formula for wavelength has been derived, applicable for a plane
of observation at any distance. A relation between the focal length and the
radius curvature is also derived for a plano-convex lens which is essentially
used as a concave mirror. Tracker, a video analysis software, freely
downloadable from the net, is employed to analyze the fringes captured using a
CCD camera. Two beams which give rise to interference fringes in conventional
Newton's rings and in the present setup are clearly distinguished.
