Determination of focal length of convex lenses using Newton's method

Abstract

The conventional Newton's method can be applied for the measurement of focal lengths of convex lenses. The advantages of this method are that it does not require the location of the principal points of the test lens for the measurement of focal length and it can be applied to the thin as well as thick lenses. But the Newton's method suffers from some of the inherent errors introduced due to systematic errors such as bench correction error and depth of focus of the optical components involved in the measurements of distances. Therefore, some modifications in this method have been suggested and the method in its modified version is applied to find out the focal length of a convex lens with better accuracies. In the suggested improvements, the use of a sufficiently illuminated target in place of cross-slit, and the travelling microscopes in place of screen, reduced the inaccuracies introduced because of the bench error and the depth of focus of the optical components involved.