The bright object AB is placed parallel and 1 distance from the screen L. The lens is converging, the focal length f is placed at the distance between the object and the screen such that AB is perpendicular to the principal axis of the lens, A lies on the principal axis.
a) find the relationship between L and f so that there are 2 positions of the lens to give a clear image on the screen? to have only 1 position and to have no position
b) find the lens positions and the relationship between the magnifications at those positions