DETERMINATION OF FOCAL LENGTH OF CONVEX LENSES USING NEWTONS METHOD

The conventional Newton's method can be applied for the measurement of focal lengths of convex lenses. The advantages of this method are tha t 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 so me of the inherent errors introduced due to systematic errors such as bench correction error and depth of focus of the optical components in volved in the measurements of distances. Therefore, some modifications in this method have been suggested and the method in its modified ver sion is applied to find out the focal length of a convex lens with bet ter accuracies. In the suggested improvements, the use of a sufficient ly illuminated target in place of cross-slit, and the travelling micro scopes in place of screen, reduced the inaccuracies introduced because of the bench error and the depth of focus of the optical components i nvolved.